What fitting is used to connect an FDNY coupling to an NST or NPT thread?

The correct choice is the adapter, which serves a crucial role in connecting an FDNY coupling to either NST (National Standard Thread) or NPT (National Pipe Thread) fittings. Adapters are specifically designed to allow for the joining of different thread types or sizes, ensuring compatibility between components that may not have matching threads or diameters.

In the context of fire service operations, using an adapter enables firefighters to couple hoses or equipment with varying thread specifications efficiently, which can be vital in emergency scenarios where seamless operation is essential. This functionality ensures that firefighters can maintain a reliable and effective water supply, regardless of the types of fittings on hand.

The other options, while relevant in different contexts, do not specifically address the need to connect differing thread types. A straight coupling is used to join two sections of the same type of hose or piping without altering the diameter or thread type, and a reducer is designed to connect pipes of different diameters but not necessarily different thread types. An elbow fitting is primarily used to change the direction of a piping run rather than to adapt differing thread types.