Final answer:
Black iron is the material used for piping natural gas due to its durability and resistance to corrosion caused by the sulfur compounds in the gas.
Step-by-step explanation:
The correct material used for piping natural gas is D. Black iron. Black iron is a particular type of steel pipe that has been treated with a black oxide coating for corrosion resistance and is recognized for its durability and safety in transporting natural gas. Unlike galvanized iron, which can flake and deposit zinc contaminants, or copper, which may react with sulfur compounds in the gas, black iron is resistant to the effects of sulfur and is not prone to corrosion caused by the natural gas, making it a safe choice for gas lines.