Final answer:
The opposition to Ngo Dinh Diem's regime in South Vietnam was primarily led by Buddhists, who took dramatic actions in response to his oppressive policies. This opposition caused the U.S. to withdraw support for Diem, leading to his assassination in a coup.
Step-by-step explanation:
The strongest and most visible opposition to President Ngo Dinh Diem's government in South Vietnam was led by Buddhists. Diem's oppressive measures against the Buddhists, such as raids on monasteries, mass arrests, and the forbidding of Buddhist flag flying, led to public demonstrations, self-immolations by monks, and ultimately widespread unrest.
The U.S., initially a supporter of Diem's anti-Communist stance, began to withdraw its support as his brutal policies became clear, leading to a coup supported by U.S. officials that ended with Diem's assassination.