That portion of the atmosphere where the makeup of the air is uniform in terms of the proportions of its component gases is termed the

Homosphere is the portion of the atmosphere where the composition of the air with regards proportion of its gases are uniform except for some trace chemicals, water vapor, Ozone O3 and pollutants.  The homosphere stretches from the earth's surface to approximately 50 miles.

Answerworking theory

The 3rd one