What was the name of the chemical that was dropped on Vietnam during the Vietnam war?

The story behind the answer

Agent Orange is correct. During the Vietnam War, the U.S. military sprayed this herbicide/defoliant to strip jungle cover and destroy crops (Operation Ranch Hand, 1962–1971). It contained dioxin (TCDD), a highly toxic contaminant linked to cancers, birth defects, and long-term environmental and health damage in Vietnam and among veterans.

Remember it this way: the “orange” name came from the orange stripe on the storage barrels—Agent Orange defoliates forests, not people directly. The other choices are classic chemical warfare agents from earlier conflicts: hydrogen cyanide (poison gas), mustard gas (blister agent), and phosgene (choking agent), none of which were standardly used in Vietnam.