What are the oxygen requirements for Part 91?

The oxygen requirements under Part 91 are indeed specified according to specific altitude thresholds. For flights at altitudes of 12,500 feet and above, if the duration of the flight at that altitude is more than 30 minutes, supplemental oxygen must be provided for the pilot and any other crew members. Additionally, at altitudes above 14,000 feet, oxygen is mandatory for the crew regardless of the duration of the flight. This regulation is in place to ensure that pilots and crew maintain adequate oxygen levels to safely operate an aircraft, as the oxygen levels in the atmosphere decrease with altitude.

The other options do not accurately reflect the requirements. For instance, while it might seem logical to require oxygen above a certain altitude, simply stating "required above 10,000 ft for all flights" would not encompass the specifics of duration and crew requirements laid out in the regulations. The assertion that "oxygen must be provided at all altitudes" is also incorrect since there are altitudes below which oxygen is not mandated under Part 91. Lastly, indicating that "no oxygen is required under Part 91" is misleading, as the regulations clearly outline circumstances where oxygen is necessary.