Which type of pipe is moderately flexible with a smooth interior surface and least susceptible to corrosion?

PVC (polyvinyl chloride) pipe is known for its moderate flexibility and smooth interior surface, which can significantly reduce friction losses when transporting water. This smoothness not only enhances flow characteristics but also minimizes the buildup of minerals and sediment deposits, improving the overall efficiency and longevity of the pipe system.

Moreover, PVC is highly resistant to corrosion, particularly when compared to materials like steel and cast iron, which are prone to rust and other forms of corrosion due to environmental exposure and water chemistry. This corrosion resistance makes PVC an ideal choice in many water distribution systems, particularly those involving potable or industrial water.

In summary, the unique combination of moderate flexibility, a smooth interior surface that promotes efficient fluid flow, and a high degree of corrosion resistance makes PVC pipe the best option among the choices provided.