If the master switch is off during an engine fire in flight, what is the next step?

Activating the fire extinguisher is the appropriate response when the engine is on fire and the master switch is off. This action is critical as it helps to suppress the fire effectively. The fire extinguisher is designed to target the flames and minimize damage to both the aircraft and the pilot's ability to manage the situation.

In the context of an in-flight engine fire, the priority is to quickly and safely extinguish the fire while maintaining control of the aircraft. Activating the extinguisher also assists in preventing the fire from spreading, which can be crucial for the safety of the flight. The other options, while they may hold merit in different scenarios, do not address the immediate need to combat the fire directly. For instance, opening cabin vents or closing cabin heat might not be as effective in a scenario where an engine fire requires prompt action.

Landing the airplane is significantly important, but addressing the fire immediately takes precedence to ensure the safety of the aircraft and its occupants during such an emergency.