what must a system do if it receives an instruction it does not understand?

To comply with the secure design principle of fail-safe defaults, what must a system do if it receives an instruction it does not understand? The system should:

A.
send the instruction to a peer server, to see if the peer can execute.

B.
not attempt to execute the instruction.

C.
close the connection, and refuse all further traffic from the originator.

D.
not launch its debugging features, and attempt to resolve the instruction.

E.
search for a close match in the instruction set it understands.