At which one of the following heights, the availability of oxygen in the air becomes almost negligible?

Correct option is D

Introduction
The composition of the atmosphere changes significantly with altitude. While the atmosphere extends much higher, at an altitude of approximately 120 km (near the upper boundary of the thermosphere), the composition becomes so diffuse that the concentration of molecular oxygen is extremely low, and the availability of breathable oxygen becomes virtually negligible. The atmosphere above this height is primarily composed of light gases like helium and hydrogen.
Information Booster
· Atmospheric Composition: Air is mostly nitrogen (about 78%) and oxygen (about 21%) near the surface.
· Oxygen Availability: While the proportion of oxygen remains constant up to about 90 km, the density and total pressure drop significantly with height.
· Negligible Height: The absolute availability of oxygen molecules becomes virtually zero around 120 km.
· Significance: This height is a boundary for the vertical extent of common atmospheric gases.
· Contrast: The maximum height at which an average human can survive for any period (the 'Karman Line' and 'Armstrong Limit') is much lower.
Additional Points
· 140 km, 160 km, and 100 km: While the air is extremely thin at 100 km (the Kármán line), the availability is considered almost negligible at the slightly higher altitude of 120 km as a more definitive upper limit for the presence of significant molecular oxygen.