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.
Humans need certain resources to survive at subsistence level -- most commonly air, food, water and usually some kind of shelter. A sustainable habitat is one in which supply of and demand for these resources are balanced.
