What is the percentage of oxygen in the air at 25,000 ft?

At sea level, the atmospheric air is composed of approximately 21% oxygen. This percentage remains relatively consistent at various altitudes; however, the partial pressure of oxygen decreases as altitude increases due to the overall decrease in atmospheric pressure. Therefore, even at 25,000 feet, the composition of oxygen in the air is still about 21%.

The other options suggest drastically lower percentages of oxygen which reflect a misunderstanding of how the composition of air works at different altitudes. While the availability of oxygen for breathing may be affected due to decreased pressure at high altitudes, the actual percentage of oxygen in the air does not change significantly. Thus, 21% accurately represents the oxygen concentration in the atmosphere at that elevation.