You have an Active Directory domain that runs Windows Server 2008 R2.
You need to implement a certification authority (CA) server that meets the following
requirements:
Allows the certification authority to automatically issue certificates
Integrates with Active Directory Domain Services
What should you do?

A.
Install and configure the Active Directory Certificate Services server role as a Standalone
Root CA.

B.
Install and configure the Active Directory Certificate Services server role as an Enterprise
Root CA.

C.
Purchase a certificate from a third-party certification authority, Install and configure the
Active Directory
Certificate Services server role as a Standalone Subordinate CA.

D.
Purchase a certificate from a third-party certification authority, Import the certificate into
the computer store of the schema master.

Explanation:
http://technet.microsoft.com/en-us/library/cc776874%28v=ws.10%29.aspx
Enterprise certification authorities
The Enterprise Administrator can install Certificate Services to create an enterprise
certification authority (CA).
Enterprise CAs can issue certificates for purposes such as digital signatures, secure e-mail
using S/MIME
(Secure Multipurpose Internet Mail Extensions), authentication to a secure Web server using
Secure Sockets
Layer (SSL) or Transport Layer Security (TLS) and logging on to a Windows Server 2003
family domain using
a smart card.
An enterprise CA has the following features:
An enterprise CA requires the Active Directory directory service.
When you install an enterprise root CA, it uses Group Policy to propagate its certificate to
the Trusted
Root Certification Authorities certificate store for all users and computers in the domain. You
must be a
Domain Administrator or be an administrator with write access to Active Directory to install
an enterprise root
CA.
Certificates can be issued for logging on to a Windows Server 2003 family domain using
smart cards.
The enterprise exit module publishes user certificates and the certificate revocation list
(CRL) to Active
Directory. In order to publish certificates to Active Directory, the server that the CA is
installed on must be a
member of the Certificate Publishers group. This is automatic for the domain the server is in,
but the server
must be delegated the proper security permissions to publish certificates in other domains.
For more
information about the exit module, see Policy and exit modules.
An enterprise CA uses certificate types, which are based on a certificate template. The
following functionality is
possible when you use certificate templates:
Enterprise CAs enforce credential checks on users during certificate enrollment. Each
certificate template
has a security permission set in Active Directory that determines whether the certificate
requester is
authorized to receive the type of certificate they have requested.
The certificate subject name can be generated automatically from the information in Active
Directory or
supplied explicitly by the requestor.
The policy module adds a predefined list of certificate extensions to the issued certificate.
The extensions
are defined by the certificate template. This reduces the amount of information a certificate
requester has to
provide about the certificate and its intended use.
http://technet.microsoft.com/en-us/library/cc780501%28WS.10%29.aspx
Stand-alone certification authorities

You can install Certificate Services to create a stand-alone certification authority (CA).
Stand-alone CAs can
issue certificates for purposes such as digital signatures, secure e-mail using S/MIME
(Secure Multipurpose
Internet Mail Extensions) and authentication to a secure Web server using Secure Sockets
Layer (SSL) or
Transport Layer Security (TLS).
A stand-alone CA has the following characteristics:
Unlike an enterprise CA, a stand-alone CA does not require the use of the Active Directory
directory service. Stand-alone CAs are primarily intended to be used as Trusted Offline Root
CAs in a CA hierarchy or when extranets and the Internet are involved. Additionally, if you
want to use a custom policy module for a CA, you would first install a stand-alone CA and
then replace the stand-alone policy module with your custom policy module.
When submitting a certificate request to a stand-alone CA, a certificate requester must
explicitly supply all identifying information about themselves and the type of certificate that is
wanted in the certificate request. (This does not need to be done when submitting a request
to an enterprise CA, since the enterprise user’s information is already in Active Directory and
the certificate type is described by a certificate template). The authentication information for
requests is obtained from the local computer’s Security Accounts Manager database.
By default, all certificate requests sent to the stand-alone CA are set to Pending until the
administrator of the stand-alone CA verifies the identity of the requester and approves the
request. This is done for security reasons, because the certificate requester’s credentials are
not verified by the stand-alone CA.
Certificate templates are not used.
No certificates can be issued for logging on to a Windows Server 2003 family domain using
smart cards, but other types of certificates can be issued and stored on a smart card.
The administrator has to explicitly distribute the stand-alone CA’s certificate to the domain
user’s trusted root store or users must perform that task themselves.
When a stand-alone CA uses Active Directory, it has these additional features:
If a member of the Domain Administrators group or an administrator with write access to
Active Directory, installs a stand-alone root CA, it is automatically added to the Trusted Root
Certification Authorities certificate store for all users and computers in the domain. For this
reason, if you install a stand-alone root
CA in an Active Directory domain, you should not change the default action of the CA upon
receiving certificate requests (which marks requests as Pending). Otherwise, you will have a
trusted root CA that automatically issues certificates without verifying the identity of the
certificate requester.
If a stand-alone CA is installed by a member of the Domain Administrators group of the
parent domain of a tree in the enterprise, or by an administrator with write access to Active
Directory, then the stand-alone CA will publish its CA certificate and the certificate revocation
list (CRL) to Active Directory.