The correct answer is Troposphere.

• The air is one of the essential conditions for the existence of life on this planet. The air is a mixture of various gases and it encompasses the earth from all sides. The air surrounding the earth is called the atmosphere.
• Troposphere:
  • It is the lowermost layer of the atmosphere.
  • The layer starts at Earth's surface and extends 8 to 14.5 km high (5 to 9 miles).
  • The troposphere contains dust particles and water vapour.
  • This is the most important layer of the atmosphere because all kinds of weather changes take place only in this layer.
  • The zone dividing troposphere from the stratosphere is known as the tropopause.

• Stratosphere:
  • ​It lies beyond troposphere, up to an altitude of 50 km from the earth’s surface.
  • The temperature in this layer remains constant for some distance but then rises to reach a level of 0°C at 50 km altitude.
  • This rise is due to the presence of ozone.
  • This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes. ​
• Mesosphere: 
  • ​This is an intermediate layer beyond the ozone layer and continues up to an altitude of 80 km from the earth’s surface.
  • The temperature gradually falls to -100°C at 80 km altitude.
  • Meteorites burn up in this layer on entering from the space.
• Thermosphere:
  • ​In the thermosphere, the temperature rises very rapidly with increasing height.
  • The ionosphere is a part of this layer.
  • It extends between 80-400 km. This layer helps in radio transmission.
• Exosphere:​
  • This is the uppermost layer of the atmosphere extending beyond the ionosphere above a height of about 400 km.
  • The air is extremely rarefied and the temperature gradually increases through the layer.
  • Light gases like helium and hydrogen float into space from here.