A pilot, flight crew, and passengers operating above _____ MSL must be provided supplemental oxygen.

The requirement for supplemental oxygen usage during flight is established to ensure the safety and well-being of all individuals on board due to the reduced oxygen levels at higher altitudes. When flying above 15,000 feet mean sea level (MSL), the Federal Aviation Administration mandates that pilots, flight crew, and passengers be provided with supplemental oxygen.

At altitudes starting from 12,500 feet MSL, pilots and crew members must use supplemental oxygen if they are flying for more than 30 minutes. However, once the aircraft reaches 15,000 feet MSL, the requirement extends to all passengers as well. This regulation is in place because, at these elevations, the atmosphere contains significantly less oxygen, which can lead to hypoxia, or oxygen deprivation, affecting cognitive and physical performance.

While options referencing lower altitudes address certain regulatory requirements, they do not encompass the comprehensive need for supplemental oxygen for all occupants, which explicitly begins above 15,000 feet MSL. Thus, option C reflects the correct altitude standard for mandatory supplemental oxygen provision for both crew members and passengers.