What did not occur in the proterozoic eon?
a. the oxygen level increased greatly in the atmosphere
b. the emergence of eukarya
c. multi-celled eukaryotic organism began to diversify
d. anaerobic bacteria kept thriving with more oxygen

C. Multi-celled eukaryotic organisms began to diversify

Explanation:

The Proterozoic is an eon in which numerous important things occurred. There were multiple organisms that appeared in this eon. They were able to perform the process of photosynthesis which resulted in gradual pilling up of oxygen into the atmosphere, thus changing the composition of the atmosphere, and setting up the terrain for the future development of other organisms. It is an eon though in which there were only single-celled organisms, while the diversification of multi-celled eukaryotic organisms occurred in the next eon, the Phanerozoic.