Which gas was NOT a major component of the Earth's early atmosphere?

The correct option is Oxygen. This answer is correct because it matches the approved learning objective to recall that volcanoes released carbon dioxide, water vapour and nitrogen in the subtopic The Earth's early atmosphere.

The correct option is Oxygen. Oxygen is correct because it directly supports the approved learning objective to recall that volcanoes released carbon dioxide, water vapour and nitrogen. 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.