Which Virtual Machine cannot be protected by the Distributed Firewall?

Which Virtual Machine cannot be protected by the Distributed Firewall?

A.
A Virtual Machine connected to a vDS Portgroup running on an ESXi 5.1 host.

B.
A Virtual Machine connected to a vSS Portgroup running on an ESXi 5.5 host.

C.
A Virtual Machine connected to a vDS Portgroup running on an ESXi 5.5 host.

D.
A Virtual Machine connected to a logical switch running on an ESXi 5.1 host.