Which of the following valves is likely to provide the greatest protection against backflow?

The reduced pressure valve is designed specifically to provide the highest level of protection against backflow in water distribution systems. Its operation involves creating a pressure differential that allows for the enforcement of a lower pressure in the downstream system compared to the supply side. This mechanism helps to prevent contaminated water from flowing back into the potable water supply.

The construction of a reduced pressure valve includes two check valves and a pressure relief valve, which helps it maintain a continuous low pressure and guards against back siphonage and back pressure conditions. As a result, this type of valve is commonly used in applications where the risk of contamination is higher, such as irrigation systems or industrial connections.

The single check valve, while it can help prevent backflow, does not offer the same level of protection, as it can be susceptible to failure under certain pressure conditions. Gate valves are primarily used for isolating sections of a system and are not designed for backflow prevention. Lastly, pressure relief valves serve to relieve excess pressure but do not prevent backflow, thus making them ineffective for backflow protection purposes.