You have a domain controller that runs the DHCP service.
You need to perform an offline defragmentation of the Active Directory database on the
domain controller.
You must achieve this goal without affecting the availability of the DHCP service.
What should you do?

A.
Restart the domain controller in Directory Services Restore Mode. Run the Disk
Defragmenter utility.

B.
Restart the domain controller in Directory Services Restore Mode. Run the Ntdsutil utility.

C.
Stop the Active Directory Domain Services service. Run the Ntdsutil utility.

D.
Stop the Active Directory Domain Services service. Run the Disk Defragmenter utility.

Explanation:
We don’t need to restart the server to defragment the AD database. We do need to stop AD
DS in order to defragment the database.

http://technet.microsoft.com/en-us/library/cc794920.aspx
To perform offline defragmentation of the directory database
1. Open a Command Prompt as an administrator: On the Start menu, right-click Command
Prompt, and then click Run as administrator. If the User Account Control dialog box appears,
provide credentials, if required, and then click Continue.
2. At the command prompt, type the following command, and then press ENTER: net stop
ntds
3. Type Y to agree to stop additional services, and then press ENTER.
4. At the command prompt, type ntdsutil, and then press ENTER.