When did cyanobacteria start producing pure oxygen?

a. after the hadean eon
b. after the proterozoic eon
c. during the hadean eon
d. during the phanerozoic eon

Cyanobacteria started producing pure oxtgen

A. After the Hadean eon

Explanation:

Cyanobacteria, otherwise called Cyanophyta, are a phylum of microscopic organisms and plastids that acquire their vitality through photosynthesis. They are the main photosynthetic prokaryotes ready to deliver oxygen.

The name cyanobacteria originates from the shade of the microorganisms. Cyanobacteria started producing pure oxygen after the Hadean eon.