D. the Vietcong and North Vietnamese demonstrates they could still fight.
This is true because the TET Offensive was a strategy or a Trap set by the Vietcong, where South Vietnam/ United States Troops And North Vietnam/ Vietcong would have a 7 day cease-fire. But The Vietcong snuck South Vietnam borders dressed as regular citizens and attacked 100+ South Vietnam Cities. Killing hundreds of Civilians and American Troops