Which of the following will not prevent backflow?

The answer is correct as a sink with a long faucet and light water pressure is unlikely to effectively prevent backflow. Backflow occurs when water flows in the opposite direction, which can lead to contamination of the potable water supply.

An air gap, which is the space between the sink drain pipe and the floor drain, is a physical separation that prevents backflow by ensuring that there is no direct connection between the potentially contaminated water and the clean water supply. The air space between the faucet and the flood rim of the sink functions similarly, allowing air to break the siphon effect that could cause backflow. A vacuum breaker is a mechanical device designed specifically to stop backflow by preventing a negative pressure from forming in the water supply.

Conversely, a long faucet with light water pressure might create a situation where water does not flow forcefully enough to prevent contaminants from being siphoned back into the system, making it ineffective in preventing backflow. Therefore, while the other options are all measures designed to prevent backflow, the situation described in the correct answer does not provide adequate protection.