The correct answer is B) variation. The difference between true heading and magnetic heading is due to magnetic variation, also known as magnetic declination. Magnetic variation is the angle between true north and magnetic north, which changes depending on the location. True heading is measured in relation to true north, while magnetic heading is measured in relation to magnetic north. The variation between the two is what causes the difference between true heading and magnetic heading. The other options are incorrect: A) deviation refers to the local magnetic disturbances caused by nearby metal objects, C) northerly error is a phenomenon that occurs in high latitudes, and D) compass error is a general term that encompasses both deviation and variation.