On the geologic time scale are marked by major, striking, and worldwide changes in the type of fossils present.
a. eras
c. epochs
b. periods
d. all of the above select the best answer from the choices provided

The right answer is a. Eras

A geological era corresponds to the second largest unit of time on the geological time scale behind the eon.

* The history of the Earth is divided into ten eras:

Eoarchéen: -4.0 to -3.6 billion years;

Paleoarchean: from -3.6 to -3.2 billion years;

Mesoarchean: from -3.2 to -2.8 billion years;

Neoarcheen: -2.8 to -2.5 billion years;

Paleoproterozoic: -2.5 to -1.6 billion years;

Mesoproterozoic: -1.6 to -1 billion years;

Neoproterozoic: from -1 to -0.541 billion years;

Paleozoic: from -541 to -252 million years ago;

Mesozoic: from -252 to -66 million years;

Cenozoic: from -66 million years to the present day.