You need to compact an Active Directory database on a domain controller that runs
Windows Server 2008 R2.
What should you do?

A.
Run defrag.exe /a /c.

B.
Run defrag.exe /c /u.

C.
From Ntdsutil, use the Files option.

D.
From Ntdsutil, use the Metadata cleanup option.

Explanation:
Reference 1)
http://technet.microsoft.com/en-us/library/cc794920.aspx
Compact the Directory Database File (Offline Defragmentation)
You can use this procedure to compact the Active Directory database offline. Offline
defragmentation returns free disk space in the Active Directory database to the file system.
As part of the offline defragmentation procedure, check directory database integrity.
Performing offline defragmentation creates a new, compacted version of the database file in
a different location.
Reference 2)
Mastering Windows Server 2008 R2 (Sybex, 2010) page 805
Performing Offline Defragmentation of Ntds.dit
These steps assume that you will be compacting the Ntds.dit file to a local folder. If you plan
to defragment and compact the database to a remote shared folder, map a drive letter to that
shared folder before you begin these steps, and use that drive letter in the path where
appropriate.
1. Open an elevated command prompt. Click Start, and then right-click Command Prompt.
Click Run as Administrator.
2. Type ntdsutil, and then press Enter.
3. Type Activate instance NTDS, and press Enter.
4. At the resulting ntdsutil prompt, type Files (case sensitive), and then press Enter.
5. At the file maintenance prompt, type compact to followed by the path to the destination
folder for the defragmentation, and then press Enter.