Your network contains an Active Directory forest. All domain controllers run Windows Server 2008 Standard.
The functional level of the domain is Windows Server 2003. You have a certification authority (CA).
The relevant servers in the domain are configured as shown below:

You need to ensure that you can install the Active Directory Certificate Services (AD CS) Certificate Enrollment
Web Service on the network.
What should you do?

A.
Upgrade Server1 to Windows Server 2008 R2.

B.
Upgrade Server2 to Windows Server 2008 R2.

C.
Raise the functional level of the domain to Windows Server 2008.

D.
Install the Windows Server 2008 R2 Active Directory Schema updates.

Explanation:
Reference:
http://technet.microsoft.com/en-us/library/dd759243.aspx
Installation requirements
Before installing the certificate enrollment Web services, ensure that your environment meets these
requirements:
A host computer as a domain member running Windows Server 2008 R2.
An Active Directory forest with a Windows Server 2008 R2 schema.
An enterprise certification authority (CA) running Windows Server 2008 R2, Windows Server 2008, or
Windows Server 2003.