All air filtration devices are generally located in the?

The correct choice, indicating that all air filtration devices are typically located in the return air plenum, is based on the function and purpose of these devices within an HVAC system. The return air plenum is responsible for collecting the air that has circulated within the occupied spaces of a building and directing it back to the air handling unit or furnace.

Air filters are essential for removing particulates, dust, and other contaminants from the air before it is heated or cooled and redistributed throughout the building. By placing the filtration devices in the return air plenum, they ensure that the air entering the HVAC system is clean, which protects the system's components and improves the overall air quality delivered to the conditioned spaces.

This arrangement also allows the filtration process to happen before the air is conditioned, contributing to system efficiency and longevity. Thus, the return air plenum is the most logical and appropriate location for air filtration devices within an HVAC setup.