For which of the following states of the engine should the oil level in the engine be checked?

The engine oil level is checked before starting a cold engine, or at least a few minutes after switching it off - answer C. With the engine running (A), the oil is circulating in the system and the reading tells you nothing, and immediately after switching the engine off (B), a large part of the oil has not yet drained from the cylinder head and passages into the sump, so the dipstick will show a level that is too low.

A correct measurement requires all the oil to have drained into the oil sump and the car to be standing on level ground. You pull out the dipstick, wipe it with a clean cloth, push it all the way in and pull it out again - the oil mark should be between the minimum and maximum marks. Too little oil risks engine seizure, and too much risks damage to the seals and the catalytic converter, so you top up in small amounts and check again. Checking the oil level every few refuels and before a longer journey is one of the simplest habits that protects you from a very expensive repair; in many newer cars the on-board computer shows the level instead of a dipstick, but the rule stays the same: the engine must be cold or switched off for a few minutes.