You have a test lab that contains 20 client computers and a server named Server1.
The client computers run Windows 7.
Server1 runs Windows Server 2008 Service Pack 2 (SP2).
You install the Key Management Service (KMS) on Server1.

You need to ensure that the client computers can successfully activate by using Server1.

What should you do?

A.
Upgrade Server 1 to Windows Server 2008 R2.

B.
Deploy five additional client computers that run Windows 7.

C.
On each client computer, run slmgr.vbs /rearm.

D.
On Server1, restart the Windows Activation Technologies service.

Explanation:
Minimum Computer Requirements

When planning for KMS activation, the network must meet or exceed the activation threshold, or the minimum number of qualifying computers that KMS requires. You must also understand how the KMS host tracks the number of computers on the network.
KMS Activation Thresholds

KMS can activate both physical computers and virtual machines. To qualify for KMS activation, a network must meet the activation threshold: KMS hosts activate client computers only after meeting this threshold. To ensure that the activation threshold is met, a KMS host counts the number of computers that are requesting activation on the network. For computers running Windows Server 2008 or Windows Server 2008 R2, the activation threshold is five. For computers running Windows Vista or Windows 7, the activation threshold is 25. The thresholds include client computers and servers that are running on physical computers or virtual machines.

Source: http://technet.microsoft.com/en-us/library/ff793434.aspx