Answer: c. endosphere
The earth atmosphere consists of four distinct layers based upon the temperature differences occurring in these layers. These includes troposphere, stratosphere, mesosphere, thermosphere and exosphere. Endosphere is not the part of the earth atmosphere, it is the term used to define the inner core of the earth. It can be distinguished into two distinct zones based upon the physical properties and composition, these zones exhibit.
1. External endosphere is in the liquid state, composed of metallic alloy, which remain in the molten state.
2. Internal endosphere remains in the soild state, composed of alloys.
Therefore, endosphere is not included as primary layers of the atmosphere.