The correct answer is option 'D': The infrared part of the solar radiation.

- Carbon dioxide (CO2) is a greenhouse gas that plays a significant role in the Earth's climate system. It is released into the atmosphere through various human activities such as burning fossil fuels, deforestation, and industrial processes.
- When solar radiation reaches the Earth's atmosphere, it consists of different wavelengths, including visible light, ultraviolet (UV) radiation, and infrared (IR) radiation.
- While a small portion of the incoming solar radiation is absorbed by CO2 in the atmosphere, the majority of it passes through to the Earth's surface.
- Once the solar radiation reaches the Earth's surface, it is absorbed and re-emitted as heat in the form of infrared radiation.
- However, some of this infrared radiation is absorbed and trapped by greenhouse gases, including CO2, in a process known as the greenhouse effect.
- CO2 molecules have the ability to absorb and re-emit infrared radiation due to their molecular structure. When infrared radiation is absorbed by CO2, it causes the CO2 molecules to vibrate and gain kinetic energy.
- As a result, the trapped energy is converted into heat, which warms the surrounding air and raises the temperature of the atmosphere.
- This phenomenon is responsible for the gradual increase in global temperatures, commonly referred to as global warming or climate change.
- The increasing concentration of CO2 in the atmosphere due to human activities enhances the greenhouse effect and amplifies the warming effect on the planet.
- It is important to note that water vapor also acts as a greenhouse gas and contributes to the warming of the atmosphere. However, CO2 plays a more significant role in long-term climate change due to its longer atmospheric lifetime and higher concentration.
- While CO2 does not absorb all solar radiations or the ultraviolet part of the solar radiation, it primarily absorbs and retains the infrared part of the solar radiation, leading to the warming of the atmosphere.

In conclusion, the increasing amount of carbon dioxide in the air is slowly raising the temperature of the atmosphere because it absorbs the infrared part of the solar radiation, contributing to the greenhouse effect and global warming.