A geologist is someone who studies geography studies the Earth studies germs

A geologist does not study germs: A geologist is focused on the rocks on earth, and germs are not rocks, but living organisms.

In a way, the other two options : "the Earth" and "geography" are both correct and they are the same: Geography is the study of the Earth, including the organisms living on it.

If we understand "the Earth" to mean only the physical Earth - then this is the correct answer.