Which device is likely installed in the discharge line to prevent liquid refrigerant from flowing back to the compressor?

A check valve is a device specifically designed to allow fluid (in this case, refrigerant) to flow in one direction and prevent backflow. In the context of a refrigeration system, installing a check valve in the discharge line serves to ensure that once the refrigerant has passed through the compressor and is moving toward the condenser, it cannot flow back into the compressor. This is particularly important because liquid refrigerant returning to the compressor can cause damage, as compressors are designed to compress gas, not liquid.

The other choices serve different purposes: a pressure gauge is used to monitor the pressure of the refrigerant within the system but does not control the direction of flow. An expansion valve regulates the flow of refrigerant into the evaporator but does not prevent backflow to the compressor. A refrigerant trap is designed primarily to collect liquid refrigerant but does not inherently prevent backflow like a check valve does. Thus, the check valve is the appropriate choice for preventing liquid refrigerant from returning to the compressor.