What type of failure is VMware Fault Tolerance designed to protect against?

What type of failure is VMware Fault Tolerance designed to protect against?

A.
A failure on the ESXi host running the primary virtual machine

B.
A failure on an application within the primary virtual machine

C.
Loss of storage connectivity to the primary virtual machine

D.
An operating system failure on the primary virtual machine

Explanation:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=display
KC&externalId=1013428