What does this mean?

Fred is scanning his network to ensure it is as secure as possible. Fred sends a TCP probe packet
to a host with a FIN flag and he receives a RST/ACK response. What does this mean?

A.
This response means the port he is scanning is open.

B.
The RST/ACK response means the port Fred is scanning is disabled.

C.
This means the port he is scanning is half open.

D.
This means that the port he is scanning on the host is closed.