Explanation: The ARPA (Automatic Radar Plotting Aid) system relies on various inputs for accurate operation, including the vessel's own speed and heading. When the ship's speed indication on the ARPA display can be set manually but does not update automatically with changes in the vessel's actual speed, it points to a failure in the primary source of speed data. The ship's **log** (speed log) is the instrument responsible for continuously measuring and providing the vessel's speed through the water to other systems, including ARPA. If the log fails or its data connection to the ARPA system is lost, the ARPA will display a "LOG OUT" message. In such a scenario, the operator might manually input an estimated speed, but this manual input will not automatically reflect changes in the vessel's true speed, precisely matching the problem description. * **A) "GYRO OUT"** indicates a failure in the gyrocompass, which provides heading (course) information, not speed. * **C) "TARGET LOST"** means the ARPA system has lost track of a specific radar target, unrelated to the vessel's own speed input. * **D) "NORTH UP"** is a display mode setting for the radar, not an error message indicating equipment failure.