What feature is essential for preventing backflow in plumbing systems?

The essential feature for preventing backflow in plumbing systems is an air gap. An air gap is a physical separation between the end of a water supply outlet and the flood level rim of a receptacle. This gap creates a barrier that prevents contaminated water from flowing back into the potable water supply due to backpressure or siphoning.

When water flows through a faucet and into a sink, for instance, if the sink were to become clogged and water rises, the air gap ensures that this water does not flow back into the supply line, thus maintaining the safety and integrity of the drinking water system.

While other options such as check valves, expansion tanks, and drain traps play important roles in plumbing systems—like controlling pressure and preventing sewage gases from entering living spaces—they do not specifically address the backflow prevention mechanism as effectively as an air gap does.