What material grants flexibility and durability to PEX piping?

The correct answer is polyethylene. Polyethylene is a type of plastic that is used in the manufacture of PEX (cross-linked polyethylene) piping. This material provides the necessary flexibility and durability that PEX is known for. The cross-linking process enhances its properties, making it resistant to temperature changes, pressure, and corrosion, which are crucial for plumbing applications. PEX's inherent flexibility allows for easier installation, especially in tight spaces, and the durability ensures a long lifespan with minimal maintenance. The other materials listed do not provide the same combination of flexibility and durability suitable for plumbing systems.