The correct answer is A) reduce absorption of refrigerant by the oil. When a refrigeration compressor is shut down, the oil in the crankcase is heated to prevent the oil from absorbing excessive amounts of refrigerant. This is important because if the oil absorbs too much refrigerant, it can lead to a reduction in the oil's lubricating properties, which could potentially cause damage to the compressor when it is restarted. The other options are incorrect because: B) Preventing refrigerant vaporization is not the primary purpose of heating the oil, C) Removing wax and gum is not necessary in this specific situation, and D) Removing entrained water is not the main reason for heating the oil in a shut-down refrigeration compressor.