What is the response from a closed TCP port which is not behind a firewall?

TCP uses a structured handshake, and its response to a connection attempt on a closed port follows a specific protocol when unobstructed by a firewall.

Why C is correct:A closed TCP port responds with a RST (Reset) and ACK (Acknowledgment) packet to terminate the connection attempt immediately. CNSP highlights this as a key scanning indicator.

Why other options are incorrect:

A:ICMP Port Unreachable is for UDP, not TCP.

B:FIN/ACK is for closing active connections, not rejecting new ones.

D:SYN/ACK indicates an open port during the TCP handshake.

References:CNSP "TCP Scanning" (Section on Port Responses) details RST/ACK as the standard closed-port response.