Which of the following is the most accurate method of determining gyrocompass error while underway?

Correct Answer: Option B — comparing the gyro azimuth of a celestial body with the computed azimuth of the body

Comparing a gyro azimuth of a celestial body to its computed azimuth (via Pub 229 or Nautical Almanac) is the most precise method. This provides an absolute reference point independent of terrestrial navigation errors, allowing the navigator to determine the exact gyro error at that specific moment.