Final answer:
c. Combustion contributes the greatest amount of CO2 to the atmosphere, while respiration, photosynthesis, and decomposition also play a part.
Step-by-step explanation:
The process that contributes the greatest amount of CO2 to the atmosphere is combustion. Combustion is the process of burning organic matter, such as fossil fuels, and it releases a significant amount of carbon dioxide into the atmosphere. This is evident in the carbon cycle, where burning of organic matter and fossil fuels are mentioned as sources of carbon dioxide in the atmosphere.
In comparison, respiration is the process through which living organisms release carbon dioxide as a byproduct, but it does not contribute to the atmosphere in the same magnitude as combustion. Photosynthesis actually removes carbon dioxide from the atmosphere as it utilizes it to make organic compounds, and decomposition also releases carbon dioxide, but it is not as significant as the amount released through combustion.