Answer: A. Cyanobacteria.
Step-by-step explanation:
Cyanobacteria are the photoautotrophs which are capable of utilizing the sunlight energy so as to produce oxygen. These are the relatives of bacteria. These organisms also have chloroplast like green plants which is an organelle responsible for absorption of sunlight. On primitive earth, the Cynobacteria were the only living species which was responsible for releasing oxygen by conducting photosynthesis. Thus these were responsible for the increase in abundance of oxygen in the atmosphere.