Final answer:
To check differential fluid levels, you need to remove the fill plug, which allows access to the fluid without needing to remove any other components.
Step-by-step explanation:
To check the fluid level in a differential, you should remove the fill plug. The fill plug is typically located on the side of the differential housing and allows access to add fluid or check the fluid level. It is not necessary to remove the driveshaft or any other components for a fluid level check, and vehicles with a differential will not have a dipstick for this purpose. Spark plugs have no relevance to checking the fluid in a differential as they are associated with the engine's ignition system. So the correct answer would be C) fill plug.