The correct answer is A) Low-pressure vapor. The refrigerant entering the compressor of a refrigeration system should be in a low-pressure vapor state. This is because the compressor's function is to take the low-pressure vapor refrigerant and compress it into a high-pressure vapor. If the refrigerant entered the compressor in any other state, such as high-pressure liquid or low-pressure liquid, it would not be properly compressed and the system would not function efficiently. The other options are incorrect because high-pressure liquid (B) is the state of the refrigerant leaving the condenser, and high-pressure vapor (C) and low-pressure liquid (D) are not the proper conditions for the refrigerant entering the compressor.