Final answer:
The majority of oxygen in the atmosphere is produced through the process of photosynthesis, whereby plants, algae, and some bacteria convert carbon dioxide and water into organic compounds, releasing oxygen as a byproduct.
Step-by-step explanation:
The oxygen in the atmosphere primarily results from the process of photosynthesis. Photosynthesis is a biological process in which energy from sunlight is harnessed by plants, algae, and certain bacteria to convert carbon dioxide and water into organic compounds, with oxygen being released as a byproduct. This process has played a major role in increasing atmospheric oxygen levels, whereas other processes like fermentation and anaerobic respiration did not contribute significantly to the oxygen content.
Although oxygen can also be produced through mechanistic processes such as photodissociation from water and nitrous oxide under the influence of high-energy radiation, and is utilized in cellular respiration and decay, the vast majority of free oxygen comes from the photosynthetic activity of organisms, including those in the oceans such as phytoplankton. The simple equation representing photosynthesis is:
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2.
Thus, option B, Photosynthesis, is the correct answer to the question as it is the main biological process that feeds atmospheric oxygen levels.