Final answer:
Polyvinyl Chloride (PVC) is the material used for lightweight, rustproof plumbing pipes due to its rigidity, durability, and resistance to degradation.
Step-by-step explanation:
The plastic used in lightweight, rustproof plumbing pipes is Polyvinyl Chloride (PVC). PVC is a synthetic plastic polymer that is quite rigid in its pure form, making it ideal for construction materials such as pipes. It's the third-most widely produced synthetic plastic, after polyethylene and polypropylene. Polyvinyl chloride is known for its durability and resistance to environmental degradation, which is why it's commonly used in plumbing systems. Other options like Teflon, polyethylene, and Bakelite are used in various applications, but PVC is preferred for plumbing pipes due to its specific properties that allow it to handle both high-pressure water and resist corrosion.