Final answer:
An air gap should be placed between the faucet and the flood rim of the sink to prevent contaminated water backflow. It provides a physical separation to prevent a siphon effect that can cause contamination of the potable water supply.
Step-by-step explanation:
The correct item that should be placed between the faucet and the flood rim of the sink to prevent backflow of contaminated water is an air gap. An air gap is a simple device that provides a physical separation between the end of a water supply outlet and the flood level of a receiving vessel, such as a sink. This separation is crucial because it prevents contaminated water from flowing backward and mixing with the potable water supply. An air gap is often a requirement in plumbing codes and serves as the most reliable backflow prevention method. It works by allowing the air to naturally break the siphon effect that could otherwise occur if, for example, the sink was to become flooded and the water level reached the outlet of the faucet.