The correct answer is cyanobacteria (ancient cyanobacteria).
The huge concentration of oxygen in the atmosphere is believed to have been initially produced by the actions of ancient cyanobacteria. They are usually observed as symbionts with a number of another group of species, like coral, lichens (fungi), Azolla (pteridophytes), angiosperms, and others.
The atmosphere of oxygen was generated by various cyanobacteria at the time of Proterozoic and Archaean periods.
