Your network contains two forests named adatum.com and litwareinc.com. The functional level of all the
domains is Windows Server 2003. The functional level of both forests is Windows 2000.
You need to create a forest trust between adatum.com and litwareinc.com.
What should you do first?

A.
Create an external trust.

B.
Raise the functional level of both forests.

C.
Configure SID filtering.

D.
Raise the functional level of all the domains.

Explanation:
Reference:
http://technet.microsoft.com/en-us/library/cc771397.aspx
When to create a forest trust
You can create a forest trust between forest root domains if the forest functional level is Windows Server 2003
or higher.