Between what altitudes must oxygen be available to at least 10 percent of the occupants, in an unpressurized airplane, other than the pilots?

Oxygen must be available for at least 10 percent of the occupants of an unpressurized airplane at altitudes above 10,000 feet through 15,000 feet MSL if flight at those altitudes exceeds 30 minutes. This regulation is in place because of the physiological effects of altitude on the human body, particularly as it relates to the availability of oxygen and the onset of hypoxia.

At altitudes above 10,000 feet, atmospheric pressure begins to drop, which reduces the amount of oxygen available to breathe. After 30 minutes at these altitudes, even healthy individuals may start to experience the effects of reduced oxygen levels, making supplemental oxygen necessary to ensure the safety and well-being of the occupants.

In contrast, the other altitudes specified in the other options either do not meet the requirements for oxygen availability or do not account for the critical time duration at higher altitudes. Such distinctions are vital for ensuring that flight operations adhere to safety regulations that protect passengers and crew.