The studies on geological history of oxygen, shows that before photosynthesis evolved, the earth's atmosphere had no free oxygen. However, the photosynthetic prokaryotes ( Archae and Bacteria), produced oxygen as a waste product. But the oxygen they produced were rapidly removed from the atmosphere by weathering of
reducing minerals, most notably iron. But there came a point in time, when these
oxygen sinks became saturated and could not capture all of the oxygen
that was produced by cyanobacterial photosynthesis and the
excess free oxygen started to accumulate in the atmosphere.
So, the correct answer is option B.