The correct answer is D. The Tet Offensive.
Explanation
Tet's offensive was a military offensive that took place during the Vietnam War, planned and executed by North Vietnam and its army against the coalition forces led by South Vietnam and the United States. Particularly, this offensive did not have positive results for North Vietnam, because it meant human and territorial losses, nevertheless, it is considered as a tactical victory because due to this incident people in the United States refused to persist in the war due to the great loss of around fourteen thousand American soldiers. This change in perception was significant for the subsequent defeat of the coalition and the victory of North Vietnam. So, the correct answer is the Tet offensive.
