Final answer:
The thermosphere's high temperatures are due to the low density of air molecules absorbing a large amount of ultraviolet radiation energy from the sun. Each molecule gains substantial kinetic energy, leading to high temperatures recorded in this layer.
Step-by-step explanation:
The thermosphere has the highest temperatures of all the atmosphere layers primarily due to the way solar radiation is absorbed. Ultraviolet radiation from the Sun strikes this layer first and hard, and since the air density is very low, the few molecules that are present, including atomic oxygen and nitrogen, absorb large amounts of energy. This leads to a significant increase in their kinetic energy, resulting in high temperatures. However, because there are so few molecules in the thermosphere, the heat would not feel hot to a human's skin.
The answer to why the thermosphere has the highest temperature of all the atmosphere layers is A. The sun's radiation strikes it first. This layer is directly exposed to the Sun's radiation, and with very few molecules to share the absorbed energy, each molecule ends up with a high amount of kinetic energy, which is expressed as a high temperature. Options B, C, and D are incorrect because the composition of the thermosphere is not primarily nitrogen molecules, it does not contain the stratosphere, and its thickness does not directly correlate with having the highest temperatures.