The correct answer is B) comparing the gyro azimuth of a celestial body with the computed azimuth of the body. This is the most accurate method of determining gyrocompass error while underway because it directly compares the gyro heading to the true heading calculated from the celestial body's position. This allows you to isolate and measure the gyrocompass error, without the influence of unknown factors like current or wind that would affect the other methods. Option A is incorrect because determining course made good from celestial fixes does not directly measure gyrocompass error. Option C is incorrect because comparing the gyro to the magnetic compass will include both gyrocompass and magnetic compass errors. Option D is incorrect because gyrocompass error can be accurately determined at sea using celestial observations.