What process is responsible for the gradual increase of oxygen levels in the atmosphere?

The correct option is Photosynthesis by algae and plants. This answer is correct because it matches the approved learning objective to describe how oxygen levels increased gradually as photosynthetic organisms became more widespread in the subtopic How oxygen increased.

The correct option is Photosynthesis by algae and plants. Photosynthesis by algae and plants is correct because it directly supports the approved learning objective to describe how oxygen levels increased gradually as photosynthetic organisms became more widespread. This belongs to the subtopic How oxygen increased 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 oxygen increased.