Explanation
The correct answer is (A) Troposphere.
The troposphere is the lowest and most dense part of the Earth's atmosphere. It extends from the surface of the Earth up to an average altitude of about 8 to 15 kilometers (5 to 9 miles). The troposphere is where weather phenomena occur, and it contains the majority of the Earth's atmospheric mass, including the air we breathe. Above the troposphere are the stratosphere, mesosphere, thermosphere, and exosphere. The ionosphere, mentioned in option (B), is a region within the upper atmosphere that is ionized by solar radiation. The hydrosphere, mentioned in option (D), refers to the entirety of Earth's water, including oceans, lakes, rivers, groundwater, and atmospheric water vapor.
