The era had many extinctions because of large variations in climate.
a. precambrian
b. mesozoic
c. paleozoic
d. cenozoic

Option C, Paleozoic Era

Explanation:

There were several climatic changes that caused extinction of species living in a palezoic era.  

a)There was continuous shifting of plates (Continental drift ) which at later stage led to the formation of Gondwana from the Rodinia supercontinent during the Cambrian Period of the Paleozoic era

b)During Cambrian Period of the Paleozoic era, the weather was warm but it was followed by series of ice ages in the Ordovician period

c)During ice ages the water level reduced as water present on the land was used for formation of glaciers.  

d)Gondwana moved south during Ordovician period and Smaller continents moved close to each other.  

e)Later on in the Devonian period , the small land masses in the north joined together to form supercontinent Euramerica. Now at this point of time expect Gondwana rest all planet was water.  

These frequent and continuous changes in the geology and geography of a regions caused the mass extinction.   Hence, Option C is correct.

b is the correct answer