The correct answer is B) determine the time of meridian transit. Apparent time is the time as indicated by the sun's position in the sky, rather than the standard clock time. The primary use of apparent time in marine navigation is to determine the time when the sun crosses the local meridian, known as the meridian transit. This information is crucial for establishing a vessel's longitude using celestial navigation techniques. The other answer choices are incorrect: A) calculating sunrise/sunset is not the primary use of apparent time, C) determining zone time requires conversion from apparent time, and D) entering an almanac does not directly utilize apparent time.