You are designing a server infrastructure to support a new stateful application.

The server infrastructure must meet the following requirements:

– Use two servers, each with two NIC cards and 32 GB of RAM.
– Provide access to the application in the event of the failure of a single server.
– Provide the ability to scale up the application.
– Minimize the attack surface of each server.
– Minimize server disk space requirements.

You need to design a server infrastructure that meets the requirements.

What should you recommend? (More than one answer choice may achieve the goal. Select the BEST answer.)

A.
Perform a Server Core installation of Windows Server 2008 R2 Standard Edition. Configure both servers in a failover cluster.

B.
Perform a Server Core installation of Windows Server 2008 R2. Configure both servers in a Windows Network Load Balancing array,

C.
Install Windows Server 2008 R2 on both servers. Use DNS Round Robin to balance the load between the servers.

D.
Install Windows Server 2008 R2 on both servers. Configure both servers in a Windows Network Load Balancing array.

Explanation:
all the requirements point to a server core install ie attack surface, disk space (no gui no need to waste disk space on it) that narrows it down to A & B
http://technet.microsoft.com/en-us/library/dd184075.aspx

NOTE: Theres some confusion over this question. in its current form no answer is 100% correct.
Answer A is the only answer to mention the edition of the OS. but Standard Edition does not support Fail Over Clustering.
the question mentions the new application will be stateful and network load balancers are not intended for the use of stateful apps. however if you look at http://technet.microsoft.com/en-us/library/cc757745%28v=ws.10%29.aspx,

Network Load Balancing and stateful connections

Application servers maintain two kinds of stateful connections:

Interclient state: A state whose updates must be synchronized with transactions performed for other clients, such as merchandise inventory at an e-commerce site.

Intraclient state: A state that must be maintained for a given client throughout a session (that can span multiple connections), such as a shopping cart process at an e-commerce site.

Network Load Balancing should not be used to scale applications that directly update interclient state, such as Microsoft SQL Server, because these applications generally were not designed to permit multiple instances to simultaneously access a shared database and synchronize updates. Instead, Network Load Balancing should be used to scale stateless front-end services, such as Microsoft Internet Information Services, that might access a shared back-end database server.

However, Network Load Balancing can be used to scale applications that manage intraclient state within a session that spans multiple connections. When client affinity is enabled, Network Load Balancing directs all TCP connections to the same cluster host. This allows session state to be maintained in host memory. Client/server applications that embed state within cookies or push it to a back-end database do not need client affinity to be maintained.

so it is possible the answer is B also the question suggests selecting the BEST answer, answer B would be the one that meets the requirements best

What Is Server Core
The Server Core option is a new minimal installation option that is available when you are deploying the Standard, Enterprise, or Datacenter edition of Windows Server 20081. Server Core provides you with a minimal installation of Windows Server 2008 that supports installing only certain server roles:

if you look at http://community.spiceworks.com/topic/110578-difference-between-nlb-and-windows-failover-cluster then consider your requirement of “Provide the ability to scale up the application” a fail over cluster wouldnt do this because it doesnt spread the load as only one server is live at any one time

Availability, scalability, and clustering technologies

Windows Server 2008 R2 provides two clustering technologies: failover clusters and Network Load Balancing (NLB). Failover clusters primarily provide high availability; Network Load Balancing provides scalability and at the same time helps increase availability of Web-based services. Your choice of cluster technologies (failover clusters or Network Load Balancing) depends primarily on whether the applications you run have long-running in-memory state:

What are failover clusters?
By using a failover cluster, you can ensure that users have nearly constant access to important server-based resources. A failover cluster is a set of independent computers that work together to increase the availability of services and applications. The clustered servers (called nodes) are connected by physical cables and by software. If one of the nodes fails, another node begins to provide service through a process known as failover. In Windows Server 2008, the changes to failover clusters (formerly known as server clusters) are aimed at simplifying cluster setup and management, making the clusters more secure and stable, improving networking in clusters, and improving how failover clusters communicate with storage. A failover cluster is a group of independent servers that are running Windows Server 2008 and working together to increase the availability of services and applications. When a failure occurs on one computer in a cluster, resources are redirected and the workload is redistributed to another computer in the cluster. You can use failover clusters to ensure that users have nearly constant access to important server-based resources. Failover clusters are designed for applications that have long-running in-memory state, or that have large, frequently updated data states. These are called stateful applications, and they include database applications and messaging applications. Typical uses for failover clusters include file servers, print servers, database servers, and messaging servers.

What are NLB clusters?

A single computer running Windows can provide a limited level of server reliability and scalable performance. However, by combining the resources of two or more computers running one of the products in Windows Server 2008 R2 into a single virtual cluster, NLB can deliver the reliability and performance that Web servers and other mission-critical servers need. Each host runs a separate copy of the desired server applications (such as applications for Web, FTP, and Telnet servers). NLB distributes incoming client requests across the hosts in the cluster. The load weight to be handled by each host can be configured as necessary. You can also add hosts dynamically to the cluster to handle increased load. In addition, NLB can direct all traffic to a designated single host, which is called the default host.

NLB allows all of the computers in the cluster to be addressed by the same set of cluster IP addresses, and it maintains a set of unique, dedicated IP addresses for each host. For load-balanced applications, when a host fails or goes offline, the load is automatically redistributed among the computers that are still operating. When a computer fails or goes offline unexpectedly, active connections to the failed or offline server are lost. However, if you bring a host down intentionally, you can use the drainstop command to service all active connections prior to bringing the computer offline. In any case, when it is ready, the offline computer can transparently rejoin the cluster and regain its share of the workload, which allows the other computers in the cluster to handle less traffic. Network Load Balancing is intended for applications that do not have long-running in-memory state. These are called stateless applications. A stateless application treats each client request as an independent operation, and therefore it can load-balance each request independently. Stateless applications often have read-only data or data that changes infrequently. Front-end Web servers, virtual private networks (VPNs), File Transfer Protocol (FTP) servers, and firewall and proxy servers typically use Network Load Balancing. Network Load Balancing clusters can also support other TCP- or UDP-based services and applications.

However if you look here http://technet.microsoft.com/en-us/library/dd443539%28v=ws.10%29.aspx at the bottom it says:

Which editions include failover clustering?

The failover cluster feature is available in Windows Server 2008 R2 Enterprise and Windows Server 2008 R2 Datacenter. The feature is not available in Windows Web Server 2008 R2 or Windows Server 2008 R2 Standard.

so we have a problem, its obvious a Core install based on the requirements, teh application being stateful means it must be a Failover Cluster but the OS edition doesnt support fail over clustering.