Which valve determines whether the pump is operating in series or pressure mode?

The transfer valve is the correct choice because it specifically controls the flow of water within the pump system, allowing the operator to switch between series and pressure modes. In series mode, the transfer valve directs the flow of water through additional pumps or stages, increasing pressure for higher elevation operations or larger flows. In pressure mode, the valve allows the pump to operate at a single stage, directly supplying the desired pressure to the discharge line.

Other valves, such as check valves, relief valves, and ball valves, serve different functions within firefighting operations. Check valves are used to prevent backflow, relief valves limit the pressure within the system to prevent damage, and ball valves are primarily used for on/off operations and not for selecting operational modes. The role of the transfer valve is pivotal in managing the operational settings of the fire apparatus, making it crucial for efficient water supply and pressure management.