The correct answer is option 1: Thermosphere. The thermosphere is the warmest layer of the atmosphere due to its proximity to the sun and the absorption of high-energy solar radiation. It is located above the mesosphere, approximately 80 kilometers above the Earth`s surface. This layer is characterized by extremely high temperatures, reaching up to 2,500 degrees Celsius or higher. However, despite the high temperatures, the thermosphere would not feel warm to humans due to the low density of molecules. The heat in the thermosphere is primarily caused by the absorption of ultraviolet and X-ray radiation from the sun, which causes the molecules to become highly energized. It is important to note that the temperature of the thermosphere can vary greatly depending on solar activity and other factors.