Your network contains a single Active Directory domain. The functional level of the forest is
Windows Server 2008. The functional level of the domain is Windows Server 2008 R2. All
DNS servers run Windows Server 2008. All domain controllers run Windows Server 2008
R2.
You need to ensure that you can enable the Active Directory Recycle Bin.
What should you do?

A.
Change the functional level of the forest.

B.
Change the functional level of the domain.

C.
Modify the Active Directory schema.

D.
Modify the Universal Group Membership Caching settings.

Explanation:
http://technet.microsoft.com/en-us/library/dd392261.aspx
Active Directory Recycle Bin Step-by-Step Guide
By default, Active Directory Recycle Bin in Windows Server 2008 R2 is disabled. To enable
it, you must first raise the forest functional level of your AD DS or AD LDS environment to
Windows Server 2008 R2, which in turn requires all forest domain controllers or all servers
that host instances of AD LDS configuration sets to be running Windows Server 2008 R2.