Which of the following increases during the hot gas defrost process?

During the hot gas defrost process, the temperature in the evaporator increases as a direct result of the introduction of hot gas from the compressor into the evaporator coils. This hot gas effectively raises the temperature within the evaporator, causing frost or ice to melt off the coils, which promotes better heat transfer and overall system efficiency.

As for the other options, during this process, the efficiency of the compressor may not necessarily improve, as it is working harder during defrost cycles. The volume of the refrigerant does not increase; in fact, as the system operates, the refrigerant is circulated through the evaporator and into other components but does not expand significantly in volume. The pressure in the expansion valve, meanwhile, typically remains lower because the expansion valve is designed to control the flow and pressure of the refrigerant entering the evaporator, which is not directly influenced by the hot gas defrost process itself. Therefore, the only correct answer is that the temperature in the evaporator increases during this defrosting process.