Carbon exists in the atmosphere in the form of Carbon Dioxide (CO2).

Explanation:

Carbon is one of the most abundant elements found on Earth and is an essential component of all living organisms. It is present in various forms in the atmosphere, one of which is carbon dioxide (CO2).

1. Carbon Dioxide (CO2):
- Carbon dioxide is a compound composed of one carbon atom bonded to two oxygen atoms.
- It is a greenhouse gas that plays a significant role in regulating the Earth's temperature by trapping heat in the atmosphere.
- Carbon dioxide is produced through natural processes such as respiration and volcanic activity, as well as human activities like burning fossil fuels and deforestation.
- It is released into the atmosphere during the combustion of fossil fuels such as coal, oil, and natural gas.
- Carbon dioxide is an essential component of the carbon cycle, which involves the exchange of carbon between the atmosphere, oceans, and land.

2. Carbon Monoxide (CO):
- Carbon monoxide is another compound that contains carbon and oxygen, but it is not as prevalent in the atmosphere as carbon dioxide.
- It is a colorless, odorless gas that is produced primarily through incomplete combustion of fossil fuels and biomass.
- Carbon monoxide is toxic to humans and other animals when present in high concentrations.
- While carbon monoxide does exist in the atmosphere, it is typically found in trace amounts and is not a significant component compared to carbon dioxide.

3. Coal:
- Coal is a solid fossil fuel that is primarily composed of carbon.
- It is formed from the remains of plants that lived and died millions of years ago.
- When coal is burned, it releases carbon dioxide into the atmosphere, contributing to the greenhouse effect and climate change.
- While coal is a significant source of carbon emissions, it is not a direct form of carbon that exists in the atmosphere.

Conclusion:
In conclusion, carbon exists in the atmosphere primarily in the form of carbon dioxide (CO2), which is a greenhouse gas and plays a crucial role in the Earth's climate system. Carbon monoxide (CO) is also present but in trace amounts, and coal is a solid fossil fuel that releases carbon dioxide when burned.