Answer:
A. source
Step-by-step explanation:
Anything that removes more carbon from the atmosphere than it absorbs is known to as a carbon sink. Examples include soil, plants, and the ocean.
A carbon source, on the other hand, is anything that adds more carbon to the atmosphere than it takes in, such as the combustion of fossil fuels or volcanic eruptions.
and since decomposing leaves gradually return carbon to the environment as carbon dioxide, the decomposition of plant mater is A CO2 source.
hope that helps :)