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 A — the refrigerant should be charged into the system as a vapor

Adding a small refrigerant top-off charge through the low side while the compressor runs requires introducing the refrigerant strictly as a vapor. This safeguards the operating compressor from catastrophic liquid slugging and mechanical damage.