The correct answer is a. Nitrogen/Oxygen captures the sun's radiation, increasing temperatures.
The thermosphere is a layer of the Earth's atmosphere that extends from about 80 km to 700 km above Earth's surface. In this layer, temperatures increase with altitude. This is because the thermosphere absorbs a lot of high-energy solar radiation, including ultraviolet and X-rays. These high-energy photons interact with the sparse molecules of nitrogen and oxygen in the thermosphere, causing these molecules to gain kinetic energy, which is observed as an increase in temperature.
This absorption of solar radiation leads to high temperatures in the thermosphere, despite the relatively low density of air in this layer. It's worth noting that while the temperatures are high, the density of particles is low, meaning there's less heat transfer, so it wouldn't necessarily feel hot to a human observer in this layer.