___________ is permitted for piping.

Wrought iron pipe is permitted for piping in propane systems due to its durability and ability to withstand high pressure, which is crucial for safely transporting propane. Wrought iron is also resistant to corrosion and can be used in both underground and above-ground applications, making it a reliable choice for various installation environments.

In contrast, the other materials listed may have restrictions or specific conditions under which they can be used. Plastic pipe, for instance, is not typically approved for high-pressure gas applications due to concerns about its structural integrity and susceptibility to damage. Copper pipe, while sometimes used in certain applications, can also be subject to limitations, especially in relation to compatibility with propane and potential reactions with other materials. PVC pipe is not suitable for gas applications at all because it can become brittle and fail under pressure, presenting significant safety risks. Hence, wrought iron stands out as the only option that meets the safety and regulatory standards for propane piping.