When checking the oil level on a reciprocating air compressor fitted with a dipstick, under what conditions should the oil level be checked?

Correct Answer: Option D — With the compressor in the off mode incapable of starting.

Checking oil levels requires the compressor to be in the 'Off' mode with a positive lockout to ensure safety. Operating machinery can cause oil splashing, leading to inaccurate readings, while the risk of accidental startup during maintenance poses a severe hazard to personnel. Always verify levels while the system is static.