Answer: a. cyanobacteria
Explanation:
Cynobacteria is considered to be the first organism on earth which was capable of producing it's own food by the process of photosynthesis. The cynobacterial strain used to exists on earth around 3.5 billion years ago. These were the first unicellular prokaryotic organisms. They had a large number of colonies. These were responsible for the liberation of large amount of oxygen in the oxygen deficient primitive earth atmosphere.
