To keep the Inmarsat-B antenna pointing at the desired satellite, regardless of the ship's position and course, it has an input from the vessel's:

Correct Answer: Option D — Gyrocompass

Inmarsat-B terminals utilize high-gain directional antennas that must remain precisely pointed at the satellite. To maintain this alignment during vessel maneuvers, the antenna control unit requires continuous, real-time heading data from the ship's gyrocompass, allowing the system to compensate for changes in the vessel's course and attitude.