Your network contains an Active Directory domain controller named DC1. DC1 runs
Windows Server 2008 R2.
You need to defragment the Active Directory database on DC1. The solution must minimize
downtime on DC1.
What should you do first?

A.
At the command prompt, run net stop ntds.

B.
At the command prompt, run net stop netlogon.

C.
Restart DC1 in Safe Mode.

D.
Restart DC1 in Directory Services Restore Mode (DSRM).

Explanation:
We don’t need to restart the server to defragment the AD database. We only need to stop
AD DS in order to defragment the database, using ntdsutil.

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.

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.