The correct answer is B) be shielded from direct sunlight. When a magnetic compass is not in use for a prolonged period, it should be shielded from direct sunlight to prevent the compass card from becoming magnetized. Direct sunlight can cause the compass magnets to become magnetized, which can affect the accuracy of the compass readings. Shielding the compass from sunlight helps maintain the integrity of the compass magnets and ensures the compass will provide accurate readings when it is put back into use. The other answer choices are incorrect. Option A is incorrect because air bubbles in the compass fluid do not need to be replaced with nitrogen. Option C is incorrect because the compensating magnets should not be removed, as they help adjust the compass for local magnetic variations. Option D is incorrect because the compass should not be locked into a constant heading, as this would prevent it from freely aligning with the Earth's magnetic field.