At what pressure altitude must at least one crewmember wear an O2 mask according to FAR 135?

In accordance with FAR 135, regulations pertaining to oxygen requirements for flight crews specify that at least one crewmember must wear an oxygen mask above certain pressure altitudes to ensure safety during operations. Specifically, the regulation states that an oxygen mask must be used when flying at or above Flight Level 250, as well as at Flight Level 350.

Choosing the option that states Flight Level 250 or 350 incorporates the requirement for both altitudes. At Flight Level 250, individuals may be at risk of experiencing the effects of hypoxia, especially if the cabin were to depressurize. Flight Level 350 introduces additional oxygen requirements due to even lower ambient pressure, which further heightens the need for supplemental oxygen.

Thus, the option correctly identifies the critical altitudes as established by federal aviation regulations, ensuring that crewmembers are prepared to maintain their own wellbeing and effectively operate aircraft in high-altitude environments where oxygen levels are insufficient.