What additional requirement is there for pressurized cabins above FL250?

The correct choice highlights that when flying at altitudes above Flight Level 250 (FL250), there is a specific requirement related to supplemental oxygen. This requirement is enacted due to the potential for hypoxia, which can occur at higher altitudes where the barometric pressure decreases and the amount of available oxygen becomes limited for both pilots and passengers.

According to aviation regulations, it is necessary for each occupant of the aircraft to have access to 10 minutes of supplemental oxygen to ensure safety in case of cabin depressurization or emergency situations. This oxygen supply affords time for pilots to descend to a safer altitude or address any malfunction that has led to a loss of cabin pressure.

The other considerations regarding pilot mask usage or oxygen supply amounts are either under- or overestimating the requirements set for pressurized cabins above FL250. It is essential for all individuals on board to be equipped properly, but the regulation specifies the need for 10 minutes of supplemental oxygen for each occupant, making this choice the correct answer.