Final answer:
The natural process that produces oxygen (O2) is 2. photosynthesis, where water is oxidized to release oxygen gas.
Step-by-step explanation:
The only natural process that produces molecular oxygen (O₂) is 2. photosynthesis. Photosynthesis is a complex series of reactions facilitated by sunlight striking chlorophyll in plants.
During these reactions, water is oxidized, resulting in the release of oxygen gas. Thus, photosynthesis is responsible for producing the oxygen necessary for aerobic respiration, which sustains much of the life on Earth.