Final answer:
The method that will prevent backflow into the drinkable water supply is the air gap, which is a physical separation that creates an open vertical gap between the two systems. This ensures that contaminated water cannot flow back into the drinkable water supply.
Step-by-step explanation:
The method that will prevent backflow into the drinkable water supply is the air gap. An air gap is a physical separation between the water supply and a potentially contaminated source. It is created by placing an open vertical gap between the two systems. This ensures that if there is a drop in water pressure or a backflow event, the contaminated water cannot flow back into the drinkable water supply.