The lower layer of the atmosphere is the troposphere.
Explanation:
A. Stratosphere: The stratosphere is the layer of the atmosphere above the troposphere. It is located between the troposphere and the mesosphere. The stratosphere contains the ozone layer, which absorbs and filters out a significant amount of the sun's harmful ultraviolet (UV) radiation.
B. Troposphere: The troposphere is the lowest layer of the atmosphere, extending from the Earth's surface up to about 10-15 kilometers (6-9 miles) in altitude. It is where weather occurs and where most of the Earth's atmospheric mass is located. This layer contains the air we breathe and is where temperature decreases with increasing altitude.
C. Air: While air is a component of the atmosphere, it is not a specific layer within the atmosphere. The atmosphere is composed of various gases, including nitrogen, oxygen, carbon dioxide, and trace amounts of other gases.
D. Hydrosphere: The hydrosphere refers to all the water on Earth, including oceans, rivers, lakes, groundwater, and water vapor in the atmosphere. It is not a layer of the atmosphere.
Therefore, the correct answer is B. troposphere.