Answer:
D. Mass extinction events separate the three eras from each other
Explanation:
The Phanerozoic represents an eon, and it is the most recent one. It is further divided into three eras, Paleozoic, Mesozoic, and Cenozoic. The division between the tree eras is because of mass extinction events. The Paleozoic and Mesozoic era are divided by the Permian-Triassic mass extinctions. The Mesozoic and Cenozoic are divided by the Cretaceous-Paleogene extinction. The Cenozoic era is still in existence, as there hasn't been a mass extinction of a magnitude big enough so far.
