Eons are the broadest category of geologic time, and we live in the phanerozoic eon. the phanerozoic eon is further divided into eras. what divides the three eras of the phanerozoic eon?
a. the appearance of carbon on earth and the appearance of atmospheric oxygen separate the three eras from each other.
b. the development of insects and the development of flying insects separate the three eras from each other.
c. the first life on earth and the first multicellular life on earth separate the three eras from each other.
d. mass extinction events separate the three eras from each other.

D. Mass extinction events separate the three eras from each other

Explanation:

The Phanerozoic is an eon, thus it is representative of the largest unit of geological time. It is further divided into three eras. The three eras of the Phanerozoic are the Paleozoic, Mesozoic, and Cenozoic. These three eras have been divided on the basis of mass extinction events. The Paleozoic ends with the mass extinction at the end of the Permian period, which is also the largest mass extinction ever, and it also marks the beginning of the Mesozoic. The Mesozoic ends with the mass extinction at the end of the Cretaceous period, occurring because of a large meteorite or comet impact, marking the beginning of the Cenozoic era. The Cenozoic era is still going, and even though there have been extinction throughout it, none of them has been of a scale big enough to be considered as a mass extinction, so until something like that happens, this era will continue to exist.