Your network contains an Active Directory forest named fabrikam.com. The forest contains
the following domains:
Fabrikam.com
Eu.fabrikam.com
Na.fabrikam.com
Eu.contoso.com
Na.contoso.com
You need to configure the forest to ensure that the administrators of any of the domains can
specify a user principal name (UPN) suffix of contoso.com when they create user accounts
from Active Directory Users and Computers.
Which tool should you use?

A.
Active Directory Sites and Services

B.
Set-ADDomain

C.
Set-ADForest

D.
Active Directory Administrative Center

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,…}