Answer:
The correct answer would be a. respiration.
In the atmosphere, carbon is present in the form of carbon dioxide. It forms the major components of organic substances or molecules. It is majorly fixed in the ecosystem by process of photosynthesis.
The number of processes or activities help to reintroduce carbon into the atmosphere. Some of them are:
Respiration: Most of the living organisms release carbon dioxide in the atmosphere through respiration. They harnessed energy from food and release carbon dioxide as the byproduct of the process.
The burning of fossil fuels also releases a huge amount of carbon dioxide in the atmosphere.
Volcanic irruptions, forests fires et cetera also release carbon dioxide in the atmosphere.
Hence, we can say that respiration helps in reintroducing carbon into the atmosphere.