Which of the following is considered a long-term carbon store?

The correct option is Limestone. This answer is correct because it matches the approved learning objective to describe limestone, coal, crude oil and natural gas as long-term carbon stores in the subtopic How carbon dioxide decreased.

The correct option is Limestone. Limestone is correct because it directly supports the approved learning objective to describe limestone, coal, crude oil and natural gas as long-term carbon stores. 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.