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 of litwareinc.com when they create user accounts by using Active Directory Users and Computers.
Which tool should you use?

A.
Active Directory Domains and Trusts

B.
Set-ADDomain

C.
Active Directory Sites and Services

D.
Active Directory Users and Computers

Explanation:
Many thanks to Camel73 for supplying this new question!
Reference:
http://technet.microsoft.com/en-us/library/cc772007.aspx
To add UPN suffixes
1. Open Active Directory Domains and Trusts.
2. In the console tree, right-click Active DirectoryDomains and Trusts, and then click Properties.
3. On the UPN Suffixes tab, type an alternative UPN suffix for the forest, and then click Add.
4. Repeat step 3 to add additional alternative UPN suffixes.