What is the result of initiating an oam mac-ping to an unknown unicast MAC address...

The oam mac-ping will fail. The MAC address must be known in the local FDB to be successful.

The request will be flooded and the only response will come from the PE node that has learned the specified MAC address on a local SAP.

The request will be flooded and the CE device with the corresponding MAC will reply.

All nodes will reply because the oam mac-ping is flooded.