The correct answer is: D. Entomology : Insects
Botany is the scientific study of plants. Ornithology is the scientific study of birds. Conchology is the scientific study of mollusks, especially snails and their shells. Pathology is the scientific study of disease. Entomology is the scientific study of insects.
In each of the four lettered pairs, the first word is the name of a scientific discipline and the second word is the subject of that discipline. The only pair that follows this pattern is D. Entomology : Insects.