Which of the following is not an approved fixed wiring method for a place of assembly?

In the context of fixed wiring methods for places of assembly, the focus is on ensuring safety, durability, and compliance with electrical codes. NM cable, also known as non-metallic sheathed cable, is typically used in residential wiring and is not suitable for installation in assembly areas. This is primarily due to its lack of suitability in high-traffic or public spaces where there is an increased risk of exposure to damage, moisture, and fire hazards.

The other options, such as metal conduit, rigid PVC, and flexible cable, are more appropriate for installations in assembly occupancies because they provide greater protection for the conductors against physical damage and environmental factors. Metal conduit offers robust protection and is often required in commercial settings to secure wiring. Rigid PVC conduit can also protect against moisture and is acceptable for certain applications in assembly areas. Flexible cables, while not typically used as a primary wiring method, can be appropriate in specific applications where movement is necessary.

Overall, NM cable's limitations in terms of durability and suitability for communal areas make it the correct answer, as it does not meet the specific requirements expected for wiring methods in places of assembly.