A corporate environment includes two Active Directory Domain Services (AD DS) forests, as
shown in the following table.

You need to ensure that users in the contoso.com domain can access resources in the
eng.fabrikam.com domain.
What should you do?

A.
Enable selective authentication.

B.
Enable forest-wide authentication.

C.
Create an external trust between contoso.com and eng.fabrikam.com.

D.
Enable domain-wide authentication.

Explanation:
http://technet.microsoft.com/en-us/library/cc816837.aspx
Creating External Trusts
You can create an external trust to form a one-way or two-way, nontransitive trust with
domains that are outside your forest. External trusts are sometimes necessary when users
need access to resources that are located in a Windows NT 4.0 domain or in a domain that
is in a separate Active Directory Domain Services (AD DS) forest that is not joined by a
forest trust.