Which gas was primarily released by volcanic activity during the formation of the early atmosphere?

The correct option is Carbon dioxide. This answer is correct because it matches the approved learning objective to explain that water vapour condensed to form the oceans as the Earth cooled in the subtopic The Earth's early atmosphere.

The correct option is Carbon dioxide. Carbon dioxide is correct because it directly supports the approved learning objective to explain that water vapour condensed to form the oceans as the Earth cooled. This belongs to the subtopic The Earth's early atmosphere 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 The Earth's early atmosphere.