Which condition requires all occupants to use supplemental oxygen?

The correct answer involves the requirement for all occupants to use supplemental oxygen when flying at altitudes above 15,000 feet. At this altitude, the partial pressure of oxygen is insufficient to sustain normal physiological function without supplemental oxygen. The Federal Aviation Administration (FAA) regulations stipulate that at altitudes above 15,000 feet, all passengers in the aircraft must be provided with and must use supplemental oxygen for the duration of the flight at that altitude. This is established to ensure the safety and health of all individuals aboard, as hypoxia can occur quickly in these conditions.

In contrast, while supplemental oxygen is recommended above 12,500 feet and is required for the pilot above 14,000 feet, these regulations do not stipulate a requirement for all occupants until reaching the 15,000-foot threshold. At sea level, there is no need for supplemental oxygen as ambient air pressure provides an adequate supply of oxygen for normal breathing.