Answer:
The most appropriate answer would be option D.
The first life appeared on earth was probably the prokaryotes. Some of them called cyanobacteria discovered the process of harnessing energy from the sun, that is, photosynthesis.
The oxygen was released as the byproduct of the process.
Consequently, the oxygen started accumulating in the oceans which were used to oxidize the iron present in the ocean. It led to the formation of iron oxide deposits in the ocean.
Once the whole iron was oxidized, the free oxygen started accumulating in the atmosphere.
Ultimately, it converted the anoxic atmosphere into the oxygen-rich atmosphere.
