When may a compulsory vessel not be allowed to leave port?

Correct Answer: Option C — When the vessel has replaced a required piece of GMDSS-related equipment but its performance has not been verified or logged.

A vessel is unseaworthy if its GMDSS equipment is not fully functional. Replacing a required component without verifying its performance through self-tests or diagnostic logs means the vessel cannot guarantee its ability to transmit distress alerts, thus failing to meet SOLAS requirements for departure from port.