Your network contains an Active Directory forest named contoso.com. The forest contains
two domains named contoso.com and child.contoso.com. The forest contains two sites
named Seattle and Denver. Both sites contain users, client computers, and domain
controllers from both domains.
The Seattle site contains the first domain controller deployed to the forest. The Seattle site
also contains the primary domain controller (PDC) emulator for both domains. All of the
domain controllers are configured as DNS servers. All DNS zones are replicated to all of the
domain controllers in the forest.
The users in the Denver site report that is takes a long time to log on to their client computer
when they use their user principal name (UPN). The users in the Seattle site do not
experience the same issue.
You need to reduce the amount of time it takes for the Denver users to log on to their client
computer by using their UPN.
What should you do?

A.
Reduce the cost of the site link between the Denver site and the Seattle site.

B.
Enable the global catalog on a domain controller in the Denver site.

C.
Enable universal group membership caching in the Denver site.

D.
Move a PDC emulator to the Denver site.

E.
Reduce the replication interval of the site link between the Denver site and the Seattle
site.

F.
Add an additional domain controller to the Denver site.

Explanation:
http://technet.microsoft.com/en-us/library/cc728188.aspx

Common Global Catalog Scenarios
The following events require a global catalog server:
(…) User logon. In a forest that has more than one domain, two conditions require the global
catalog during user authentication:
1. When a user principal name (UPN) is used at logon and the forest has more than one
domain, a global catalog server is required to resolve the name.
2. (…)