What does 14 CFR Part 91 state regarding supplemental oxygen for flight crews?

The correct answer indicates that supplemental oxygen must be used when flying above 12,500 feet MSL for more than 30 minutes. This regulation is in place to ensure the safety of flight crews by preventing hypoxia, which can occur at higher altitudes where the atmosphere is thinner, reducing the amount of available oxygen. The requirement helps to support cognitive functions and maintain pilot performance during flight operations.

At altitudes over 12,500 feet, the risk of hypoxia increases, so regulations mandate the use of supplemental oxygen after a certain duration to mitigate these risks. This is crucial for maintaining alertness and decision-making ability, which is essential for flight safety.

The other options do not accurately reflect the regulations under 14 CFR Part 91 regarding supplemental oxygen usage for crew members, either overestimating or underestimating the requirements for different altitudes and situations.