When a valve displays whether it's open or closed, which feature does it include?

The feature that allows a valve to display whether it's open or closed is known as an indicator post. This type of apparatus provides visual confirmation of the valve's position, which is crucial for operators to ensure that the fire service mains are functioning correctly. An indicator post can be particularly important in emergency situations where quick decisions need to be made regarding water flow, as it enhances the safety and efficiency of fire protection systems.

In contrast, the other options serve different functions: a pressure gauge measures the pressure of the water flow within the system, a flow meter quantifies the amount of water flowing through the system, and a backflow preventer is designed to prevent water from flowing backward through the system. While these components are vital for the overall functionality and protection of the fire service mains, they do not provide information on the open or closed status of a valve.