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

The use of plastic as a material for gas piping is not acceptable due to its properties and the safety regulations surrounding gas systems. Gas lines must be made from materials that can withstand the pressure and temperature fluctuations commonly associated with gas distribution. Plastic does not have the same durability and resistance to heat and pressure as metal materials like black iron, copper, and flexible corrugated stainless steel tubing, which are specifically designed and approved for gas piping applications.

Additionally, plastic can degrade over time when exposed to certain environmental factors or chemicals, which further compromises the integrity of a gas line. This poses a significant safety risk, as leaks can occur if the plastic piping fails. Thus, maintaining strict standards for gas piping materials is crucial for ensuring the safety and reliability of gas delivery systems, which is why plastic is prohibited in this context.