The correct option is D Ichthyology
Herpetology is the branch of biology that deals with the study of amphibians and reptiles. Ichthyology is the branch of biology that deals with the study of the fishes. It includes the study of species, history, behaviour in ecosystem and their growth pattern.
Mammalogy is the branch of biology that deals with the study of mammals.
Ornithology is the study of birds.
Thus, the correct answer is option B.