To prevent automatic water supply from leaking out through the FDC, what must be installed in the FDC line?

The correct choice is the installation of a check valve in the FDC (Fire Department Connection) line to prevent automatic water supply from leaking out. A check valve is designed to allow flow in one direction while preventing backflow. In the context of a fire protection system, this is crucial for ensuring that water remains in the system when not in use and does not accidentally flow back out.

When the fire department connects their hoses to the FDC, they need to draw water into the system. The check valve helps ensure that when pressure is released (for example, when hoses are disconnected or the fire department's operations are finished), water does not leak back out of the system and into the supply line. This not only optimizes the efficiency of the fire protection system but also helps maintain its integrity and pressure readiness for future incidents.

In contrast, other assemblies like the double-check backflow preventer, reduced pressure detector assembly, and double detector check assembly serve different purposes. They are primarily used to protect the drinking water supply from contamination or backflow in various applications, rather than specifically to stop leakage in the context of an FDC.