Which piping material is prohibited for use in underground gas piping?

The correct choice is PVC, as it is prohibited for use in underground gas piping. This prohibition is based on safety and code compliance requirements. PVC, or polyvinyl chloride, does not have the necessary durability and resistance to physical damage, temperature variations, and the chemical properties of natural gas. It can become brittle over time and is susceptible to degradation from UV light and ground moisture.

In underground applications, it is essential for piping materials to withstand the harsh conditions they may encounter, including soil movement, moisture, and potential impacts. Other materials, such as steel and cast iron, are generally more durable and suitable for underground applications, while aluminum alloy is sometimes used but is not common for gas piping. Thus, PVC's unsuitability for underground gas piping is why it is the correct answer.