Which of the following cannot serve as supply, return, exhaust, relief, or ventilation air ducts?

The correct answer highlights the limitation of using corridors as air ducts for supply, return, exhaust, relief, or ventilation purposes. Corridors are primarily designed for movement and accessibility, and they are not typically constructed or designated to handle air flow in the manner required for ductwork.

Using corridors as air ducts could compromise the building's safety and accessibility requirements because they could obstruct egress paths in case of an emergency. Additionally, the design and construction of corridors do not usually meet the criteria for ductwork, such as maintaining appropriate airflow, insulation, and fire safety standards. Therefore, this option is clearly defined by its functional limitations.

In contrast, the other choices—rooms, plenums, and closets—can all serve as functional spaces for air movement within a mechanical system. Plenums, for example, are specifically designed to distribute air to various ducts. Understanding these distinctions is critical for anyone working within HVAC design, ensuring compliance with the North Carolina Mechanical Code and maintaining both efficiency and safety in building ventilation systems.