What action is critical if you suspect a fire on board the aircraft?

Activating the fire extinguisher is the critical action if you suspect a fire on board the aircraft because it is the immediate measure that can help suppress the flames and mitigate danger. Fire on an aircraft can escalate rapidly, making it essential to try to control it as soon as it is detected. The fire extinguisher is specifically designed for this purpose, providing a direct response to the emergency.

Other options, while they may seem relevant in certain scenarios, do not address the immediate need to combat the fire. Redirecting the aircraft towards the nearest airport or reducing airspeed may provide a means of land but won't mitigate the active threat of fire while airborne. Opening all vents could inadvertently feed oxygen to the fire, exacerbating the situation. Therefore, activating the fire extinguisher is the most critical step to ensure safety on board in the event of a suspected fire.