Answer:
D. It showed that the Vietcong could mount a successful attack on South Vietnamese and were not close to defeat despite what the US government told the American people.
Step-by-step explanation:
Despite the gigantic military U.S. deployment and buildup in Vietnam, the Vietnamese communist forces managed to surprise American and allied South Vietnamese troops by launching an offensive in 100 cities in January/February 1968, the Tet Offensive; that was what America´s and the world´s eyes could see.
The offensive was a military setback for the Communists (they weren´t able to take and hold in one city), but it was a huge PR and image success that influenced the mood of the American public opinion. It showed the chaos and brutality of the war, its uncertainty; it was not clear the U.S. was winning the war, as American leaders wanted people to believe. It reinforced opposition to war and the pacifist movement in American cities.
Tet is the Vietnamese Lunar New Year.