What type of test is required when a valve is returned to service to ensure it is open?

The appropriate test that is required when a valve is returned to service to ensure it is open is a functionality test. This test is specifically designed to confirm that valves are operating correctly, which includes ensuring they are in the proper position (open or closed) as intended.

When a valve is returned to service, the functionality test verifies its operational status, ensuring that it will perform as required in the event of a fire. This is crucial for maintaining the integrity and effectiveness of the fire protection system.

While other tests such as pressure tests and flow tests have their significance in the context of overall system performance and water supply verification, they do not specifically focus on the mechanical operation of a valve. The drain test primarily assesses the system's ability to drain water and is not utilized to confirm that a valve is open. Thus, the functionality test is the most appropriate method to validate that a valve has been correctly positioned and is ready for service.