The correct answer is (a) Nervous system
Explanation: Nervous system is most affected by the organochlorine insecticide i.e. the DDT. The group of insecticide produces its activity by affecting the functioning of the nervous system. The molecule interferes with the activity of the ionic permeability of nerve cell membranes, whereas, Circulatory system, heart, digestive system is not certainly affected by the organochlorine insecticide like DDT and Gamma BHC.