Which event most directly caused increasing instability in South Vietnam in 1955?

In 1955, Ngo Dinh Diem held a referendum to decide whether he or former emperor Bao Dai should lead South Vietnam. The vote was widely regarded as rigged in Diem’s favor, undermining trust in his government and increasing political instability. Thus, the rigged presidential election was the most direct cause among these choices.

AI-written and checked against the verified answer.