A fuel pressure warning switch contacts close and warning light is turned on when:

The fuel pressure warning switch is designed to monitor the fuel pressure within the aircraft's fuel system. When the fuel pressure drops below specified limits, it indicates that there may be an issue with fuel delivery, which could impact engine performance or lead to engine failure.

In this situation, the warning switch activates by closing its contacts, which in turn triggers the warning light to alert the pilot to the critical condition. Monitoring fuel pressure is essential for safe flight operations, and immediate awareness of inadequate fuel pressure allows for prompt corrective actions to be taken, such as switching to a backup pump or performing emergency procedures.

Other scenarios, like the fuel flow stopping or a specified quantity of fuel passing through, do not directly correlate with the fuel pressure levels. Similarly, turning off the boost pump might not necessarily indicate a drop in fuel pressure immediately; it depends on system design and the conditions within the fuel lines. Hence, the correct response pertains specifically to the fuel pressure being critically low, prompting the warning alert.