What does a single vertical magnet placed underneath the compass in the binnacle compensate for?

Correct Answer: Option C — deviation caused by the vessel's inclination from the vertical

A heeling magnet, positioned vertically under the compass, compensates for deviation caused by the vessel's inclination. When a ship heels, the vertical soft iron and permanent vertical magnetism create a force that shifts the compass card, which this magnet counteracts to maintain accuracy during rolling or heeling.