Which FAA FAR $($Federal Aviation Regulation$)$ outlines the requirements and recommendations for aircrew and passengers affected by the possible lack of oxygen in the cockpit and cabin?

Which FAA FAR $($Federal Aviation Regulation$)$ outlines the requirements and recommendations for aircrew and passengers affected by the possible lack of oxygen in the cockpit and cabin?

Part $135$

Part $121$

Part $131$

Part $117$