A car's air conditioning system blows warm air despite the compressor being activated. What could be causing this issue?

In the scenario where a car's air conditioning system blows warm air even with the compressor activated, the most likely cause can be linked to a fault in the compressor. The compressor plays a crucial role in the functioning of the air conditioning system by compressing the refrigerant and circulating it through the system. If the compressor is faulty, it cannot effectively compress the refrigerant, which results in a lack of proper cooling. Consequently, the system will blow warm air instead of the desired cool air.

While other options might suggest potential issues within the system, they do not pinpoint the primary function that may be leading to the failure of cooling. For instance, a clogged expansion valve or a damaged condenser could impair the system's ability to regulate temperature, but they would not directly prevent the compressor from circulating refrigerant. Low refrigerant levels can also cause warm air, yet it’s often a symptom of other issues such as a leak rather than a direct malfunction of the compressor itself.

Understanding the role of the compressor helps clarify why a fault in this component would manifest as warm air blowing from the A/C vents, making it the most reasonable answer in this context.