What is best described as a device used to prevent back-flow in a water system?

The best option for describing a device used to prevent back-flow in a water system is the back-flow-prevention assembly. This assembly is specifically designed to ensure that water flows in one direction and prevents any return flow, which could lead to contamination of the potable water supply. It typically consists of multiple components, such as check valves and possibly other devices, that work together to protect against back-flow, particularly in situations where the water supply might be at risk due to environmental factors or sudden pressure changes in the system.

Other devices like a flow regulator or a pressure relief valve serve different purposes; for instance, flow regulators manage the rate of water flow, while pressure relief valves are used to relieve excess pressure within a system to prevent damage. Although check valves can also prevent back-flow, they do not offer the comprehensive protection and reliability that a complete back-flow-prevention assembly does.