Answer: A. Photosynthesis by cynobacteria
On earth, the atmosphere was warm and reducing and oxygen was absent. The cynobacteria or blue green algae were the first organisms on earth, which were able to produce oxygen by the process of photosynthesis. They utilized sunlight, water and carbon dioxide from the external environment to produce carbohydrates along with the oxygen. With the increase in abundance of the cynobacteria population on early earth, oxygen abundance also increased.
Therefore, photosynthesis by cynobacteria is hypothesized to be the early source of free oxygen in the Earth's atmosphere.