Your network contains two Active Directory forests named contoso.com and nwtraders.com.
The functional level of both forests is Windows Server 2003. Contoso.com contains one
domain. Nwtraders.com contains two domains.
You need to ensure that users in contoso.com can access the resources in all domains. The
solution must require the minimum number of trusts.
Which type of trust should you create?

A.
external

B.
forest

C.
realm

D.
shortcut

Explanation:
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. Creating a forest trust between two root domains with a
forest functional level of Windows Server 2003 or higher provides a one-way or two-way,
transitive trust relationship between every domain in each forest. Forest trusts are useful for
application service providers, organizations undergoing mergers or acquisitions,
collaborative business extranets, and organizations seeking a solution for administrative
autonomy.