Answer :
Answer:
It is D. Ngo Dinh Diem's refusal to hold elections.
Explanation:
It's correct on Edmentum.

The refusal of Ngo Dinh Diem to hold elections was the event that triggered the civil war in Vietnam.
What is the Vietnam civil war?
The war was a long conflict that splitted the communist government of North Vietnam against South Vietnam
The war was caused by the refusal of Ngo Dinh Diem to hold elections.
Therefore, the Option D is correct.
Read more about Vietnam civil war
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