Which is a fire hose appliance carried on a fire apparatus?

The gated wye is a fire hose appliance commonly carried on fire apparatus, and it serves a critical function in firefighting operations. This device allows firefighters to split a single hose line into two separate lines, enabling them to supply multiple hose crews from one water source. The "gated" aspect means that it features individual valves that can be opened or closed, allowing for better control over water flow to each line.

This is particularly useful in large-scale incidents where multiple streams of water are needed simultaneously, as it enables flexibility in water distribution without the need for additional water sources. By using a gated wye, firefighters can swiftly adapt to changing situations in the field, effectively managing the supply of water to various parts of the operation.

In contrast, the other options mentioned do not serve this specific function or are not standard appliances found on fire apparatus. For instance, a hose coaster is more about organizing and managing hose lines rather than splitting supply. The term "wye thief" might refer to a similar concept but is less recognized as a standard appliance. A water keeper generally refers to a specific function rather than a distinct appliance used for hose connections.