Which wiring method is not permitted in ducts transporting environmental air?

The correct answer highlights that liquidtight flexible metal conduit is not permitted in ducts transporting environmental air due to safety and regulatory considerations. In such ducts, the primary concern is preventing any potential contamination of the air supply, which could occur with the use of flexible conduits. Typically, these conduits are designed for wet or harsh environments and are not intended for air handling spaces where the air will be circulated throughout a building.

In contrast, nonmetallic sheath cable, rigid PVC conduit, and mineral insulated cable are generally permissible in these applications. Nonmetallic sheath cable is designed for general indoor use and does not present a risk of contamination. Rigid PVC conduit is non-corrosive and does not leak materials that could affect air quality. Mineral insulated cable has a solid, fire-resistant construction and can safely be used without posing a risk to the air being transported. Thus, the regulations prioritize maintaining air quality and safety while determining the appropriate wiring methods for use in environmental air ducts.