Which of the following would prevent the application of a Host Profile to an ESX Host
(Choose Two)?

A.
The host has not been placed into Maintenance Mode

B.
The host is an ESX 3.5 Host

C.
The host has multiple profiles attached

D.
The host is an ESXi Host

Explanation:
Vmware Knowledge Base KB 1010732

Applying a Host Profile to a host or cluster

Note: The host must be in maintenance mode before applying it to a profile.

The screenshot below shows the error obtained whilst trying apply a Host Profile to asystem not in maintenance mode.
Host profiles from ESX 3.5 U3 to ESX 4.0 U1

As others have stated, you can only use host profiles with ESX 4 hosts. So you can create
a profile from an existing ESX 4 host and then apply it to other existing ESX 4 hosts