The correct option is It is the highest layer..
Explanation
The Earth's atmosphere is divided into five distinct layers based on thermal properties and composition: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere. The Exosphere represents the outermost boundary of the atmospheric envelope.
- It is the densest layer. is incorrect: The Troposphere is the densest layer, containing the majority of the atmosphere's mass. Atmospheric density decreases exponentially with altitude; consequently, the Exosphere is the least dense layer, consisting of extremely rarefied air.
- It is the highest layer. is correct: The Exosphere is the highest layer of the atmosphere. It extends from the top of the Thermosphere (approximately 600-700 km above sea level) outward to about 10,000 km, where it gradually merges with interplanetary space.
- It contains the ozone layer. is incorrect: The Ozone layer is primarily located within the Stratosphere, specifically between 15 and 35 km above the Earth's surface. It is not found in the Exosphere.
- It is the coldest layer. is incorrect: The Mesosphere is the coldest layer of the atmosphere, with temperatures dropping as low as -90°C. While the Exosphere has high kinetic temperatures due to solar radiation, it would not feel hot to a human observer because the particle density is too low to transfer heat effectively.
Key Takeaway: The Exosphere is the transitional zone between Earth's atmosphere and outer space, characterized by extremely low density and the presence of light gases like hydrogen and helium.