The correct answer is Troposphere Explanation & Key Takeaways: Troposphere • The troposphere is the lowest portion of Earth’s atmosphere and contains approximately 70% of the total air in the atmosphere’s mass and 99% of its water vapour and aerosols. Hence statement 4 is correct. • The average depth of the troposphere is approximately 17 km in the middle latitudes. • The characteristic features of the Troposphere are its great density. • In addition to nitrogen and oxygen, carbon dioxide, and water vapour (nearly all of the water vapour contained in the atmosphere is concentrated in the troposphere) and numerous particles of various origin • The thickness of the Troposphere is maximum at the equator, deeper in the tropics, up to 20 km, and shallower near the polar regions, at 7 km in summer, and indistinct in winter. • In India, it is taken to be around 16 Kilometers. • The thickness of the troposphere and consequently the atmosphere is maximum at the equator • The boundary between the troposphere and stratosphere, called the tropopause, is a temperature inversion. Stratosphere • The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere. • It is called stratosphere because it is stratified in temperature, with warmer layers higher up and cooler layers farther dow • The stratosphere is free from the violent weather changes in the Troposphere. So, it is preferred by commercial airliners. Mesosphere • it extends above the stratosphere up to a height of about 80 km. • It is the coldest layer of the atmosphere. • The mesosphere is directly above the stratosphere. • The stratosphere and mesosphere are collectively termed as " Middle atmosphere " • Within the mesosphere, temperature decreases with increasing height. Thermosphere • The thermosphere starts just above the mesosphere and extends to 600 kilometers (372 miles). {Between 40 km-400km} • Aurora and satellites occur in this layer.