When a refrigeration system is being topped off with a small amount of refrigerant through the low side with the compressor running, what should be done?

Correct Answer: Option C — the refrigerant should be charged into the system as a vapor

Refrigerant must enter the low side as a vapor to prevent liquid slugging, which causes catastrophic mechanical damage to reciprocating compressor valves and pistons. Throttling through the suction service valve ensures complete vaporization before entering the compressor cylinder.