The correct answer is C) 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 damage to the compass. Direct sunlight can heat up the compass housing and affect the magnetization of the compass needle, causing it to become less accurate over time. Shielding the compass from sunlight helps maintain the integrity of the compass and ensures it remains properly calibrated when needed for navigation. The other answer choices are incorrect because: A) locking the compass into a constant heading would not prevent damage from lack of use, B) removing the compensating magnets is not necessary for long-term storage, and D) replacing air bubbles with nitrogen is not a standard procedure for compass maintenance during periods of non-use.