One who studies insect life:

Answer: Option C

The one word substitution is Entomologist.

Geologist : one who studies the physical structure and substance of the earth, their history, and the processes which act on them..
Zoologist : one who studies behaviour, structure, physiology, classification, and distribution of animals.
Botanist : one who studies study of the physiology, structure, genetics, ecology, distribution, classification, and economic importance of plants.
Entomologist : One who studies insect life.