What is the minimum altitude at which pilots must use supplemental oxygen when flying?

The use of supplemental oxygen for pilots is governed by regulations to ensure safety during flight at higher altitudes where the atmosphere contains less oxygen. Specifically, supplemental oxygen must be used when flying above 12,500 feet MSL (Mean Sea Level) up to and including 14,000 feet MSL. This altitude is crucial because at these levels, a pilot may begin to experience the physiological effects of reduced oxygen availability, which can impair cognitive and physical performance.

In the context of this question, option B is correct since the regulation specifies that at 12,500 feet, pilots must have supplemental oxygen available for flights lasting longer than 30 minutes. Above 14,000 feet, pilots are required to use supplemental oxygen at all times, which underscores the importance of maintaining proper oxygen levels during flight to ensure the safety of both the pilot and the passengers.