Which device prevents supplied water from being pumped back into the public water system?

The device specifically designed to prevent supplied water from being pumped back into the public water system is the backflow preventer. This device plays a crucial role in maintaining the safety and integrity of water supplies. It ensures that any water that has flowed through a pipe does not reverse direction and re-enter the public water system, which could contaminate the drinking water supply with pollutants or hazardous materials from the private system.

In contrast, while a check valve can prevent backflow in a system, its primary function is to allow fluid to flow in one direction and prevent it from flowing backward within the same system, rather than safeguarding the public water supply. A flow switch, on the other hand, is used to detect the flow of water and activate a connected system or alarm; it does not directly prevent backflow. A pressure gauge measures the pressure of the water within a system but has no function related to backflow prevention.

In essence, the backflow preventer is designed explicitly for protecting public water systems from any potential contamination, making it the correct answer.