Name one major component of an HVAC system.

The compressor is indeed a major component of an HVAC system. It plays a crucial role in the refrigeration cycle by compressing the refrigerant gas and raising its pressure and temperature. This process is essential for the effective transfer of heat, allowing the system to cool or heat air for indoor use. The compressor's operation is fundamental for ensuring the overall efficiency and functionality of the HVAC system, as it enables the cooling or heating elements to work properly.

While the thermostat and air filter are also important components—regulating temperature and maintaining air quality respectively—they do not play as central a role in the core function of heat transfer as the compressor does. The inclusion of "All of the above" acknowledges the significance of all listed components, but the compressor specifically is a defining element of the HVAC's operation, making it the correct choice as a major component.