Why do scientists study index fossils?

Consider this plant cell which organelle is labeled g

All individuals have two alleles for a given trait. according to mendel's these alleles are passed down one each from both mother and father.

What did not occur in the proterozoic eon a)the oxygen levels increase greatly in the atmosphere,b)the emergence of the eukarya , c)muti cella’s eukaryotic organisms began to diversify,d)anaerobic bacteria kept thriving with more oxygen