A flight crewmember must be able to don and use a quick-donning oxygen mask within how many seconds?

The correct choice is 5 seconds because regulatory standards set by aviation authorities require a flight crewmember to don and use a quick-donning oxygen mask in an emergency situation as swiftly as possible. Quick-donning masks are specifically designed for rapid deployment to ensure that the aircrew can maintain adequate oxygen supply during emergencies such as cabin depressurization or smoke in the cockpit.

The 5-second timeframe is a standard to ensure that crewmembers can promptly resume their critical flight duties while ensuring their safety and the safety of passengers. This quick response time is crucial in high-stress situations where every second counts in maintaining the safety of the aircraft.

Therefore, flight crews must practice and be trained to accomplish this task efficiently within the established timeframe, reinforcing the importance of preparedness and rapid response in aviation operations.