Your network contains an Active Directory domain named contoso.com. All domain controllers run Windows
Server 2008 R2. The functional level of the domain is Windows Server 2008 R2. The functional level of the
forest is Windows Server 2008.
You have a member server named Server1 that runs Windows Server 2008.
You need to ensure that you can add Server1 to contoso.com as a domain controller.
What should you run before you promote Server1?

A.
dcpromo.exe /CreateDCAccount

B.
dcpromo.exe /ReplicaOrNewDomain:replica

C.
Set-ADDomainMode -Identity contoso.com -DomainMode Windows2008Domain

D.
Set-ADForestMode -Identity contoso.com -ForestMode Windows2008R2Forest

Explanation:
Reference:
http://technet.microsoft.com/en-us/library/understanding-active-directory-functional-levels.aspx
After you set the domain functional level to a certain value in Windows Server 2008 R2, you cannot roll back or
lower the domain functional level, with one exception: when you raise the domain functional level to Windows
Server 2008 R2 and if the forest functional level is Windows Server 2008 or lower, you have the optionof rolling
the domain functional level back to Windows Server 2008. You can lower the domain functional level only from
Windows Server 2008 R2 to Windows Server 2008. If the domain functional level is set to Windows Server
2008 R2, it cannot be rolled back, for example, to Windows Server 2003.