Final answer:
Never operate a hermetic compressor when the system is in a vacuum to prevent potential damage and overheating.
Step-by-step explanation:
You should never operate a hermetic compressor when the system is in a vacuum (c). Operating a compressor in these conditions can lead to a range of issues like causing the compressor motor to overheat due to the lack of suction cooling, or even potentially damaging the compressor since there is no refrigerant to compress. This is a critical maintenance tip for ensuring the longevity and proper function of refrigeration and air conditioning systems which involve a cycle of compression, condensation, and evaporation of the refrigerant.