General Science question with answer

Q.The final Agent Orange raid in Vietnam took place in 1970 areas have begun to bloom again. But 19 years after the end of the war, it seems plain that Agent Orange is killing and maiming human beings, something it never intended to do. The apparent toxic fallout from those clouds is a crop of human miseries including cancers, miscarriages and birth defects that may persist for decades. The offensive substance referred to in this quotation is