Answer;
- C. Oxygen
Explanation;
-Cyanobacteria, or blue-green algae, are the microbes that were responsible for the release of oxygen that we use today in the atmosphere. These microbes conduct photosynthesis using solar energy form the sun, water and carbon dioxide to produce carbohydrates and oxygen.
-As the algae would produce O2 as a waste product of photosynthesis, the free oxygen would combine with the iron ions to form magnetite (Fe3O4), an iron oxide (Banded iron formations).