Which of the following piping materials is not approved for gas piping?

Cast iron is not approved for gas piping due to its brittle nature, which can result in failures or leaks under pressure or impact. Gas piping requires materials that can withstand environmental stress, pressure fluctuations, and the potential for mechanical stress without breaking. Unlike more flexible materials like polyethylene or durable materials like black iron and galvanized steel, cast iron is prone to cracking, especially when subjected to temperature changes or heavy loads.

While other materials listed are generally approved for gas piping in various applications, cast iron's limitations in terms of safety and structural integrity make it unsuitable for transporting gas. This aligns with building codes and safety regulations that prioritize the use of reliable and resilient materials in gas distribution systems.