Only humans can remove carbon from this carbon sink / reservoir.

A. Plants

B. Animals

C. Atmosphere

D. Fossil fuels

The answer is D. Fossil fuels are a carbon reservoir that is only accessible to humans. Plants and animals are also carbon based but they are accessed by other plants and animals naturally. The atmosphere is a large carbon reservoir as well but it can be accessed by plants. Think how plants require carbon in the air to live. So the answer must be D.

I think it was D. Hopefully I'm right