Your network contains two Active Directory forestsnamed contoso.comand nwtraders.com.
The functional levelof both forestsis Windows Server 2003.
Contoso.comcontains one domain.
Nwtraders.comcontains two domains.
You need to ensure that users in contoso.com can accessthe 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:
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. 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, andorganizations seeking a solution for administrative
autonomy.