The correct option is Exosphere.
Explanation
The Earth's atmosphere is stratified into five concentric layers based on temperature profiles and composition: the Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
Option Analysis:
- Thermosphere is incorrect: The Thermosphere is located directly below the exosphere. While the air density is low, it is characterized by a rapid increase in temperature with altitude due to the absorption of solar radiation. It is not the final boundary merging with space.
- Exosphere is correct: The Exosphere is the outermost layer of the atmosphere. It extends from the top of the thermosphere (exobase) outward. In this layer, the air is extremely rarefied, and gas molecules are so sparse that they can travel hundreds of kilometers without colliding. This layer gradually fades into the vacuum of outer space without a distinct boundary.
- Ionosphere is incorrect: The Ionosphere is an overlapping region defined by electrical properties (ionization) rather than temperature. It spans parts of the mesosphere and thermosphere but does not constitute the highest physical layer of the atmosphere.
- Mesosphere is incorrect: The Mesosphere lies above the stratosphere and below the thermosphere. It is known as the coldest layer of the atmosphere and is the region where meteors typically burn up. It is well below the outer limits of the atmosphere.
Key Takeaway: The Exosphere represents the transitional zone between Earth's atmosphere and interplanetary space, characterized by extremely low atmospheric density and the escape of lighter gases like hydrogen and helium into space.