On a fire department connection, what device allows water to flow in one direction?

The device that allows water to flow in one direction on a fire department connection is the clapper valve. This type of valve is specifically designed to prevent backflow, which is critical in fire protection systems to ensure that water only flows towards the intended direction—in this case, into the firefighting system rather than back into the supply line.

Clapper valves typically have a hinged plate (the clapper) that opens to allow water to flow in one direction but closes when water attempts to flow backward. This functionality is essential in fire suppression systems to maintain pressure and ensure water is available when needed while also preventing contamination of the water supply.

Other devices listed, like the butterfly valve, check valve, and ball valve, serve different purposes. A butterfly valve is a flow regulation device more than a check valve; while a check valve does prevent backflow, it does so using a different mechanism without the hinged design specific to clapper valves. Ball valves are primarily for on/off control rather than preventing backflow. Thus, the unique function of the clapper valve in maintaining unidirectional flow in firefighting systems makes it the correct choice for this question.