What are used to prevent water from flowing out of the fire protection system back into the water supply system?

The correct choice is backflow preventers. These devices are specifically designed to prevent the reverse flow of water, ensuring that contaminated water from the fire protection system does not flow back into the potable water supply. Backflow can occur when there is a pressure difference between the fire protection system and the water supply, which could lead to serious health hazards if contaminated water is introduced into the public water system.

Check valves also play a role in preventing reverse flow, but they are not explicitly designed for use in drinking water systems and may not provide the same level of protection against contamination as backflow preventers. Isolation valves are used to shut off sections of a pipe system for maintenance but do not address backflow issues. Pressure relief valves help manage excess pressure within a system but do not prevent water flow back into the supply system. Thus, backflow preventers are the appropriate device for maintaining the integrity of the water supply against potential contamination from fire protection systems.