The space above a hung ceiling used for environmental air-handling purposes is an example of ______.

The space above a hung ceiling that is designated for environmental air-handling purposes is referred to as a plenum. This classification arises from its function as a space where air is distributed throughout a building’s HVAC (Heating, Ventilation, and Air Conditioning) system. According to the National Electrical Code (NEC), specifically section 300.22(C), any area that is used for the movement of environmental air is recognized as a plenum.

In this context, the term "plenum" designates the air space that can facilitate the flow of warmed or cooled air to various parts of a building. This has implications for electrical installations within these spaces, as there are specific requirements related to wiring methods and materials to ensure safety and efficiency while mitigating risks such as fire hazards.

The other options address related concepts but do not accurately define the space above the hung ceiling in the context provided. Understanding this definition is crucial for compliance with codes and effective design in electrical and HVAC systems.