When a magnetic compass is not in use for a prolonged period of time it should __________.

Correct Answer: Option C — be shielded from direct sunlight

Prolonged exposure to direct sunlight can cause the liquid inside the compass bowl to expand, potentially damaging the internal components or creating bubbles. Shielding the compass when not in use preserves the integrity of the fluid and the sensitivity of the card, ensuring accuracy when the instrument is next required.