Final answer:
The best method to prevent backflow is with an air gap, a simple yet effective physical separation that ensures backflow does not contaminate the potable water system.
Step-by-step explanation:
The best way to prevent backflow in plumbing is with an air gap. An air gap is the most effective means of preventing backflow because it is a physical separation between the end of the water supply outlet and the flood level of the fixture. This space ensures that any backflow of water, possibly contaminated, will not siphon back into the potable water system. It's a simple and reliable method that doesn't require moving parts. Vacuum breakers and check valves are mechanical devices that can also prevent backflow by allowing water to flow in only one direction, but they can fail if not maintained properly. A pressure regulator is a device meant to control the water pressure, which isn't specifically designed to prevent backflow.