Why is oxygen present in the Earth's atmosphere?

Oxygen is present in the Earth's atmosphere primarily due to the process of photosynthesis carried out by plants, algae, and certain bacteria, which convert carbon dioxide and water into glucose and oxygen using sunlight. This biological activity has been happening for billions of years, leading to a significant accumulation of oxygen in the atmosphere.

While some oxygen is released through other processes, like the decay of organic matter, the predominant source contributing to atmospheric oxygen is indeed photosynthesis. This process not only supplies the oxygen needed for the respiration of most life forms but also plays a crucial role in Earth’s carbon cycle, impacting climate and environmental systems.

Other options mention processes that do not significantly contribute to the atmospheric levels of oxygen in the way that photosynthesis does. For instance, the burning of fossil fuels releases carbon dioxide rather than producing oxygen, and while decay contributes to the cycling of gases, it does not create atmospheric oxygen in substantial amounts.