What indicates that the system is currently flowing foam?

The correct choice indicates that the system is actively dispensing foam when the control module's 'on' light flashes. This flashing light is a visual cue that signifies foam is being delivered as part of the firefighting operations. The fluctuation in the light can often be designed to alert the operator to the status of the foam system, creating an easily understandable signal for those managing the equipment.

The other options could confuse operators in identifying the foam flow status. A continuously lit 'off' light would typically suggest that the foam system is inactive or shut down. An alarm from the control module might indicate a problem or malfunction, rather than confirming foam flow. Lastly, while a pressure reading can be useful for gauging system performance, it does not specifically confirm whether foam is flowing at that moment without additional context. Thus, the flashing 'on' light provides the clearest indication of current foam flow.