What must occur to abort the APU start if a fuel leak is detected?

For an APU start to be aborted due to a detected fuel leak, the correct course of action is for the fire guard to communicate the situation and take appropriate actions. This response is crucial because a fuel leak poses significant safety risks, including the potential for a fire or explosion. The fire guard's role is to monitor safety conditions surrounding the APU and ensure that emergency procedures are initiated promptly.

Immediate action is necessary to mitigate the risk associated with a fuel leak. The fire guard's responsibilities typically include alerting crew members and initiating fire suppression measures if needed, making their communication and proactive response vital for ensuring safety.

Other options do not adequately address the implications and urgency of handling a detected fuel leak. Simply notifying the pilot may not be sufficient, as immediate corrective action is required to secure the area and prevent a potential emergency. Shutting down other systems could be part of an emergency procedure, but the first step must always involve acting on the fuel leak specifically. Waiting for a specified period before taking action contradicts the principle of immediate response that is essential in scenarios where safety is at risk. Thus, the fire guard's prompt communication and action stand out as the most effective and necessary response to manage such a critical situation.