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The toxic substance referred to in the provided quote is the infamous "Agent Orange." Let`s explain each option for students:
Option 1: DDT used as insecticide. DDT is indeed a harmful chemical but it was not the substance used in the Vietnam War referred to in the quote.
Option 2: A complex mix of herbicides and weedicides used to increase agricultural output in South Vietnam under the US aid programme. Though this option also implies harmful chemicals, it inaccurately suggests they were used for agricultural purposes, not for warfare as was the real case.
Option 3: A complex mix of DDT and other insecticides used as aerial sprays for protection against malaria and other tropical diseases. This choice is incorrect, there was no indication that the substance was used for disease prevention.
Option 4: Dioxin used as defoliants - this aligns with the quote`s reference to "Agent Orange," an herbicide and defoliant chemical used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. This is why option 4 is the correct answer.