At what cabin altitude will the crew oxygen masks automatically deliver 100% oxygen under pressure?

The correct cabin altitude at which the crew oxygen masks automatically deliver 100% oxygen under pressure is 35,000 feet. This altitude is significant because it exceeds the physiological limits for humans, where the risk of hypoxia (insufficient oxygen) increases dramatically. Above this threshold, the body cannot effectively utilize the available oxygen in the cabin air, making it critical for crew members to have access to supplemental oxygen.

At 35,000 feet, the aircraft's pressurization system and safety protocols are designed to protect the well-being of the crew. The automatic switching to 100% oxygen ensures that they can maintain cognitive function and physical coordination in the event of cabin depressurization or significant altitude. Lower altitudes, such as those presented in the other options, typically would not trigger this system, as the risks of hypoxia are manageable without additional oxygen.