Which type of hose fitting would be used to connect a smaller hose line to a larger supply line?

The correct choice is a reducer, as it is specifically designed to connect hose lines of different diameters. In firefighting operations, situations often arise where a smaller diameter hose needs to be connected to a larger supply line to optimize water flow or adapt to available gear. A reducer provides a smooth transition from the larger hose fitting to the smaller one, ensuring minimal turbulence and effective water delivery.

In contrast, an adapter is generally used to connect two hoses of different types or threads, rather than sizes, which makes it less suitable for this particular need. An extender typically serves to lengthen an existing hose without changing its diameter and is not useful for size transitions. Couplings are used to join two lengths of the same diameter hose together but do not facilitate connecting hoses of different sizes. Therefore, the reducer uniquely fulfills the requirement of bridging different hose diameters.