Without each occupant having supplemental oxygen, no person may operate a civil aircraft of U.S. registry above a maximum cabin pressure altitude of

• A. 12,000 ft
• B. 15,000 ft
• C. 18,000 ft
• D. 20,000 ft

Answer: (B)

Oxygen is the key factor here: at higher cabin altitudes the air contains less available oxygen, so without supplemental oxygen the risk of hypoxia rises quickly. The rule is that, if every occupant does not have supplemental oxygen, a civil aircraft operating under U.S. registry cannot have a cabin pressure altitude above 15,000 feet. This is the cutoff where oxygen must be provided for each person aboard. Below this altitude, you could operate without oxygen for limited durations, but once you exceed 15,000 feet, you must supply oxygen to everyone on board. The other numbers don’t reflect this single-regulation limit: the 15,000-foot mark is the specific threshold for requiring oxygen for all occupants when no one is equipped with supplemental oxygen.