The precambrian eon is separated from the phanerozoic eon in the fossil record because of

the number of earthquakes occurring at that time.

the appearance and disappearance of volcanoes.

the creation of new ocean ridges.

the sudden appearance of organisms.

The Precambrian eon is separated from the Phanerozoic eon in the fossil record because of the sudden appearance of organisms.

Explanation:

Precambrian eon is the initial existence on earth that can be predicted by the study of rocks and remaining of fossils. It has been seen as the existence of skeleton in this era, before this era no such things has been observed, even the development of sexual reproduction.

The Phanerozoic era comes after the Precambrian era, more advanced kinds of fossils of animals had been observed in this era.  Phanerozoic era period of life on earth. Large organisms sudden appearance are seen in the Phanerozoic era i.e. First dinosaurs, mammals, fishes, angiosperms,first vertebrates etc.

answer; (a) medicalized;