Final answer:
Oxygen is required for both passengers and crew when the cabin pressure altitude exceeds 15,000 ft to prevent hypoxia and altitude sickness due to reduced oxygen levels at high altitudes.
Step-by-step explanation:
The O2 requirement for cabin pressure altitudes above 15,000 ft is the provision of supplemental oxygen for both passengers and crew members. This is due to the decreased concentration and partial pressure of oxygen at higher altitudes, leading to potential hypoxia. At these elevations, the physiological mechanisms in the body may not be able to compensate rapidly enough for the reduced availability of oxygen, thereby increasing the risk of altitude sickness if supplemental oxygen is not supplied. Oxygen saturation levels at high altitudes can be dangerously low, resulting in a depleted venous oxygen reserve and the potential for severe health complications.
Therefore, the correct answer to the question is C) Oxygen required for passengers and crew. This is essential to prevent symptoms of oxygen deprivation and to ensure the safety and well-being of all individuals on board the aircraft during flight.