<em>Cyanobacteria started producing pure oxtgen </em>
<em>A. After the Hadean eon </em>
<u>Explanation:</u>
<em>Cyanobacteria, otherwise called Cyanophyta,</em> are a phylum of microscopic organisms and plastids that acquire their vitality through photosynthesis. They are the main photosynthetic prokaryotes ready to deliver oxygen.
<em>The name cyanobacteria originates from the shade of the microorganisms. Cyanobacteria started producing pure oxygen after the Hadean eon.</em>
