Which of the following must be in the ON position while airborne?

The correct choice indicates that the Fuel Boost Pump Switchlights need to be in the ON position while airborne. This is crucial because fuel boost pumps are responsible for maintaining adequate fuel pressure to the engines, especially during phases of flight where engine performance is critical. If these pumps are not operating properly or are turned off, it can lead to fuel starvation, engine surging, or even engine failure, which compromises flight safety.

Maintaining these pumps in the ON position ensures a reliable fuel supply to the engines under varying altitude and atmospheric pressure conditions encountered during flight. This aligns with operational safety protocols and best practices aimed at safeguarding aircraft performance and preventing potential in-flight emergencies.