What must be installed on a water heater for service, maintenance, and repair?

The correct answer emphasizes the necessity of a drain valve on a water heater for effective service, maintenance, and repair. A drain valve is vital for allowing homeowners or technicians to remove sediment that can accumulate within the tank. This sediment buildup can lead to decreased efficiency, potential overheating, and even tank failure if not properly managed. The drain valve provides convenient access for draining water from the tank, facilitating routine maintenance and prolonging the lifespan of the water heater.

While the other components mentioned, such as a safety shut-off, expansion tank, and pressure relief valve, are important for overall safety and functionality, they do not serve the same maintenance and service role as a drain valve. The pressure relief valve, for example, is essential for preventing excessive pressure buildup within the tank, and the expansion tank helps manage thermal expansion in a closed system, but neither directly addresses the need to maintain the water heater's internal cleanliness and functionality like a drain valve does.