The correct answer is D) heading. The compass deviation changes as the vessel changes heading because the magnetic fields on the ship, such as those from the ship's steel structure, engines, and electrical equipment, can interfere with the compass and cause it to deviate from true north. As the ship's heading changes, the relationship between the ship's magnetic fields and the compass changes, resulting in a change in the compass deviation. The other options are incorrect because the compass deviation is not directly affected by the vessel's geographical position (A), speed (B), or longitude (C). These factors may indirectly affect the compass deviation, but the primary driver of changes in compass deviation is the vessel's heading.