The Earth's atmosphere is primarily composed of Nitrogen (approx. 78%), Oxygen (approx. 21%), and Argon (approx. 0.9%). The remaining gases make up a small fraction.
Water vapour is a highly variable component. Its concentration changes significantly with location, altitude, and temperature.
While Nitrogen and Oxygen percentages are relatively constant, water vapour can range from nearly 0% in cold, dry regions to a maximum of about 4% by volume in hot, humid conditions near the surface.
Given these comparisons, 0.04 is the most appropriate value representing the approximate mixing ratio or concentration of water vapour in the Earth's atmosphere, acknowledging its variability.

In the above figure, the curves A, B and C denote____, ____ and ____ distributions, respectively.

In the above diagram, A, B and C represent, respectively, the tropospheric concentrations of