What type of air does the HVAC system supply when it is fully operational?

The HVAC system, when fully operational, is primarily designed to provide comfortable indoor air conditions, which often involves cooling air during warmer months. The correct choice, representing the typical function of many HVAC systems, is that they supply cool air, particularly in air conditioning systems.

In cooling mode, these systems remove heat from the indoor air and circulate cooler, conditioned air back into the space. This process includes the removal of humidity, which contributes to the overall comfort level in a building. Additionally, while filtered air might be a characteristic of the air supplied, it does not accurately capture the essential operational focus of most HVAC systems regarding temperature control.

Hot air can be supplied during heating cycles, and stale air typically refers to air that has not been adequately refreshed or conditioned, which does not align with a fully functional HVAC system's purpose. Thus, the core function of supplying cool air, especially in comfort settings during warmer temperatures, solidifies this choice as the correct answer.