Which plumbing system component prevents backflow and siphonage?

The component that effectively prevents backflow and siphonage in plumbing systems is known as a check valve. These devices allow fluid to flow in one direction only, which is crucial for maintaining a system's integrity and preventing contaminants from entering clean water supplies. When water attempts to flow backward due to changes in pressure or other factors, the check valve closes, thereby stopping any backflow and protecting the system from potential contamination.

In this context, traps, vents, and air gaps serve different functions. Traps are installed to provide a water seal that prevents sewer gases from entering buildings; however, they do not effectively stop backflow from occurring. Vents allow air to enter the plumbing system to maintain neutral pressure and facilitate drainage but are not designed to prevent backflow in liquid flows. Air gaps, while effective for preventing contamination by creating a physical separation between the water supply and potential source of contamination, do not serve specifically to prevent backflow in the same way that check valves do.

Thus, check valves are specifically engineered for the prevention of backflow and siphonage, making them the correct choice in this situation.