Which best describes a non-return valve?

A non-return valve, commonly known as a check valve, is designed to prevent the backward flow of fluids. Its primary function is to allow flow in one direction while blocking reverse flow, which is crucial in various systems to maintain pressure and prevent back-siphonage or contamination. This mechanism is essential in applications like water supply systems, sewage treatment, and other fluid transport systems, where uninterrupted and unidirectional flow is required.

The function described in the correct choice aligns with the fundamental purpose of a non-return valve, making it the best description among the given options. The other choices incorrectly characterize the valve’s operation or intended use; for example, gas flow in both directions would not be supported by a non-return valve, nor is it designed primarily for performance under high pressure or for regulating flow in a throttling manner, as those functions pertain to different types of valves.