Introduction:
Global warming is the long-term increase in Earth's average surface temperature due to the buildup of greenhouse gases in the atmosphere. These gases trap heat from the sun and prevent it from escaping back into space, leading to a gradual rise in temperature. Carbon dioxide (CO2) is one of the main greenhouse gases responsible for global warming.

Explanation:
1. Carbon dioxide (CO2):
Carbon dioxide is the main contributor to global warming. It is released into the atmosphere through various human activities such as burning fossil fuels (coal, oil, and natural gas), deforestation, and industrial processes. CO2 molecules trap heat radiated from the Earth's surface, causing the greenhouse effect. This results in the warming of the planet over time.

2. Other greenhouse gases:
While carbon dioxide is the most significant greenhouse gas, there are other gases that also contribute to global warming. These include methane (CH4), nitrous oxide (N2O), and fluorinated gases. Methane is released during the production and transport of coal, oil, and natural gas. Nitrous oxide is emitted from agricultural and industrial activities. Fluorinated gases are synthetic compounds used in various industrial applications.

3. Nitrogen (N2) and oxygen (O2):
Nitrogen and oxygen are the two main components of air, making up about 78% and 21% of the atmosphere, respectively. Unlike greenhouse gases, nitrogen and oxygen do not have the ability to trap heat and contribute to global warming. They are considered inert gases, meaning they do not actively participate in the greenhouse effect. Therefore, increasing the concentration of nitrogen or oxygen in the atmosphere would not lead to global warming.

Conclusion:
Among the constituents of air, carbon dioxide is the primary driver of global warming. It is released into the atmosphere through human activities, particularly the burning of fossil fuels. Other greenhouse gases, such as methane and nitrous oxide, also contribute to global warming but to a lesser extent. Nitrogen and oxygen, which make up the majority of air, are inert gases and do not directly contribute to the greenhouse effect. Therefore, the correct answer is option 'D' - Carbon dioxide.