An administrator is in the process of upgrading ESX 3.5 Hosts to vSphere. vCenter Data
Recovery has been installed and configured to back up the virtual machines in the
datacenter. However, after a scheduled backup it appears that not all of the virtual
machines selected were successfully backed up. What is most likely the cause for this
issue?

A.
The virtual machines which were not backed up are running unsupported Operating
Systems.

B.
The affected virtual machines did not have the VDR agent installed

C.
Some of the virtual machines are on the ESX 3.5 hosts.

D.
Some of the virtual machines were running during the backup window.

Explanation:
VMware Data Recovery Frequently Asked Questions (FAQ), April 2009.
VMware Data Recovery is not backward compatible to ESX 3.5 or earlier hosts.
Part 3: Secure VMware ESX/ESXi (21 questions).