Gigantic mammals (mammoths, giant sloths, etc.) characterize these times.

a. Pleistocene,

b. Holocene,

c. Pliocene,

d. Cretaceous

The answer is a. Pleistocene.

A group of large animals that lived during the Pleistocene is called Pleistocene megafauna. This group included mammoths, giant sloths, mastodons, cave bears, saber-toothed cats, etc. The extinction of these animals happened also in Pleistocene. The reasons for their extinction could be hunting by the humans, climate changes, or impact of asteroides.