Final answer:
The air gap method is used to prevent backflow 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 consists of an unobstructed vertical space that prevents the flow of contaminated water back into the clean water supply.
For example, in plumbing, an air gap is commonly found in the drain line of a sink or a dishwasher, where the drainpipe is higher than the flood level rim of the sink or dishwasher.