Which of the following is not true about listening ports?

This is true - a listening port is held open by a running application to accept inbound connections, making it a factually correct statement and not the answer.

Seeing traffic on a known port does not guarantee identification of the associated service, because attackers and legitimate software can run services on non-standard ports, use port tunneling, or encapsulate traffic to disguise its true nature. Port numbers are conventions enforced only by configuration, not by the protocol itself, so traffic on port 80 may not be HTTP. Reliable service identification requires deeper packet inspection beyond port number alone.

This is true - valid port values range from 1 to 65535 (with 0 reserved), so this statement is accurate and not the answer.

This is true - TCP port 80 is the well-known standard port for HTTP Internet traffic, making this factually correct and not the answer.