Which of the following gases is present in only small proportions in the Earth's atmosphere?

The correct option is Carbon Dioxide. This answer is correct because it matches the approved learning objective to recall that the Earth's atmosphere is about 20 percent oxygen in the subtopic The proportions of different gases in the atmosphere.

The correct option is Carbon Dioxide. Carbon Dioxide is correct because it directly supports the approved learning objective to recall that the Earth's atmosphere is about 20 percent oxygen. This belongs to the subtopic The proportions of different gases in the 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 proportions of different gases in the atmosphere.