Geologists have divided earth's history into time units, which are largely based on
a. oral histories.
b. fossils.
c. written correspondences.
d. climates.

I think the correct answer from the choices listed above is option B. Geologists have divided Earth's history into time units, which are largely based on fossil records. In the context of Earth's history, time is generally divided on the basis of biotic composition which are recorded by the fossils being discovered.