Which of the following is NOT a commonly used type of pipe in plumbing systems?

Steel is less commonly used in modern plumbing systems compared to the other options listed. While steel pipes were historically prevalent, especially in older installations, their use has declined significantly due to various factors. Steel is prone to rust and corrosion, which can compromise the integrity of plumbing systems over time.

In contrast, copper, PVC, and PEX are widely utilized materials in contemporary plumbing. Copper is favored for its durability and resistance to bacteria, while PVC is popular for its lightweight properties and resistance to corrosion. PEX has gained popularity for its flexibility and ease of installation. Therefore, steel, while still found in some applications, is considered less common in today's plumbing practices.