Which of the following is NOT a method of backflow prevention?

The correct choice is the Pressure Relief Valve because it is primarily used to release pressure from a system to prevent damage, rather than specifically preventing backflow. Backflow prevention focuses on stopping the reverse flow of water in plumbing systems, which can lead to contamination.

An air gap involves a physical distance between two pipes, ensuring that there is no direct connection that could allow backflow. A check valve is designed to allow fluid to flow in one direction only, preventing backflow by sealing in the opposite direction. A backflow preventer is a device specifically designed for this purpose, incorporating mechanisms that automatically stop backflow under various conditions.

In summary, while pressure relief valves serve an important function in controlling pressure, they do not address the prevention of backflow, which is the focus of the question.