Your network contains an Active Directory forest named contoso.com. The forest contains
six domains.

You need to ensure that the administrators of any of the domains can specify a user
principal name (UPN) suffix oflitwareinc.com when they create user accounts by using Active
Directory Users and Computers.
Which tool should you use?

A.
Active Directory Administrative Center

B.
Set-ADDomain

C.
Active Directory Sites and Services

D.
Set-ADForest

Explanation:
We would use the following command to achieve this:
Set-ADForest -UPNSuffixes @{Add=”contoso.com”}
Reference 1)
http://technet.microsoft.com/en-us/library/dd391925.aspx
Creating a UPN Suffix for a Forest
This topic explains how to use the Active Directory module for Windows PowerShell to
create a new user principal name (UPN) suffix for the users in a forest. Creating an
additional UPN suffix helps simplify the names that are used to log on to another domain in
the forest.
Example
The following example demonstrates how to create a new UPN suffix for the users in the
Fabrikam.com forest:
Set-ADForest -UPNSuffixes @{Add=”headquarters.fabrikam.com”}
Reference 2)
http://technet.microsoft.com/en-us/library/ee617221.aspx
Set-ADForest Modifies an Active Directory forest.
Parameter
UPNSuffixes Modifies the list of user principal name (UPN) suffixes of the forest. This
parameter sets the multi-valued msDS-UPNSuffixes property of the cross-reference
container. This parameter uses the following syntax to add remove, replace, or clear UPN
suffix values.
Syntax:
To add values:
-UPNSuffixes @{Add=value1,value2,…}