DDT (Dichlorodiphenyltrichloroethane) was the first chlorinated organic insecticide to be widely used. It was synthesized in 1874 by Othmar Zeidler and later popularized for its effectiveness as an insecticide in the 1940s.
DDT became widely used to control mosquito populations and other pests, especially in agriculture and for controlling diseases like malaria and typhus.
Thus, the first chlorinated organic insecticide is DDT, making (D) the correct answer.