You work as the enterprise administrator at Domain.com. The Domain.com network consists of a single Active Directory domain named Domain.com. The company runs Windows Server 2008 on all domain controllers on the network.
At present the domain functional level as well as the forest functional level is set to Windows 2000 native mode. To ensure productivity management wants you to make sure that the UPN suffix for Domain.com is accessible for user accounts within the network. You thus need to determine the first step that should be executed to accomplish this.

A.
The Domain.com forest functional level should be raised to Windows Server 2003 or Windows Server 2008.

B.
The Primary DNS Suffix option in the Default Domain Controllers Group Policy Object (GPO) should be changed to Domain.com.

C.
The new UPN suffix should be added to the forest.

D.
The Domain.com domain functional level should be raised to Windows Server 2003 or Windows Server 2008.