Why is oil added to the refrigerant in a vapor-cycle air conditioning system?

In a vapor-cycle air conditioning system, adding oil to the refrigerant is primarily done to lubricate the compressor. The compressor is a critical component of the system that compresses the vapor refrigerant, and it operates under high pressure and temperature conditions. Lubricating the compressor with oil ensures smooth operation and reduces wear and tear on the moving parts. This lubrication is essential for maintaining the longevity and efficiency of the compressor, preventing mechanical failures that could lead to system malfunctions.

Other options, while they may relate to different aspects of air conditioning systems, do not accurately describe the role of oil in the refrigerant. For instance, increasing cooling capacity or assisting with condensation does not involve adding oil; these functions are related to refrigerant properties and system design. Additionally, reducing air pressure is not a relevant context for oil addition in this system, as the oil's role is strictly linked to lubrication rather than pressure regulation.