If you are not using supplemental oxygen what are the restrictions for between 12,500 - 14,000' MSL?

The correct choice reflects the established guidelines for altitude exposure without the use of supplemental oxygen in aviation. When flying at altitudes between 12,500 and 14,000 feet mean sea level (MSL), regulations specify that pilots are limited to a maximum duration of 30 minutes of exposure. This guideline is put in place to mitigate the risk of hypoxia, which can occur at higher altitudes due to the decrease in atmospheric pressure and the corresponding reduction in available oxygen.

This limit is crucial for maintaining safety and ensuring that pilots do not experience significant symptoms of altitude sickness or impaired cognitive function. At altitudes above 14,000 feet, the requirement for supplemental oxygen becomes more stringent, and longer exposure times without it may lead to serious physiological effects. In contrast, other durations mentioned in the options either exceed the recommended exposure time or state that no restrictions apply, which does not account for the physiological risks associated with flying at these altitudes.