How do carbonates contribute to the geological carbon cycle?

The correct option is By forming long-term carbon stores in rocks. This answer is correct because it matches the approved learning objective to explain that marine organisms used carbonates to make shells and skeletons in the subtopic How carbon dioxide decreased.

The correct option is By forming long-term carbon stores in rocks. By forming long-term carbon stores in rocks is correct because it directly supports the approved learning objective to explain that marine organisms used carbonates to make shells and skeletons. This belongs to the subtopic How carbon dioxide decreased within The composition and evolution of the Earth's atmosphere, so the explanation must stay tied to that curriculum context. The other options are incorrect because they either do not answer this learning objective, use a vague statement, or move away from How carbon dioxide decreased.