Which of the following is the greatest threat posed by backups?

A.
A backup is the source of Malware or illicit information.

B.
A backup is unavailable during disaster recovery.

C.
A backup is incomplete because no verification was performed.

D.
An un-encrypted backup can be misplaced or stolen.

Explanation:
If the data written on the backup media is properly encrypted, it will be useless for anyone without the key.
http://resources.infosecinstitute.com/backup-media-encryption/