At what altitude must the crew use oxygen for the entire duration of the flight?

The correct altitude at which the crew must use oxygen for the entire duration of the flight is 14,000 feet. This requirement is based on Federal Aviation Administration (FAA) regulations, which stipulate that when operating at altitudes above 14,000 feet, flight crew members are required to use supplemental oxygen throughout the duration of their flight. This regulation is in place due to the decrease in atmospheric pressure and oxygen density at higher altitudes, which can impair cognitive function and physical performance. Therefore, to ensure safety and maintain operational effectiveness, it is essential for the flight crew to have a continuous supply of oxygen when flying at or above this elevation.

While other options present altitudes where oxygen use may be required under certain circumstances, the specific requirement for continuous use during the entire flight duration starts at 14,000 feet.