I'm not sure that your four multiple choice answers provide the best choices for your question. For certain, the Tet Offensive attacks all took place in South Vietnam, so answer "B" can be ruled out. However, the other three answers are all at least partly correct. During the first stage of the Tet Offensive, the North Vietnamese forces attacked more than 100 towns and cities in the south, including major strikes against Khe Sanh, Hue and parts of Saigon. Rural areas were not a major focus, so answer "D" is your second weakest answer. The siege of Khe Sanh proved to be the bloodiest of the battles, with more than 8000 Viet Cong troops being killed, but the attacks there may have been more of a diversion to draw allied forces from the more populated areas in the lowlands.
The way this question and the potential answers are worded makes it hard to tell what the right answer is supposed to be.
A is probably not right because the attack on Khe Sanh started before the actual Tet Offensive did. There was a major attack and siege of Khe Sanh but this started in early January. The Tet Offensive is typically said to have started on January 31st, after Khe Sanh was already under attack.
However, it is hard to say that either C or D is correct (B is clearly wrong). As the quote below shows, the attacks happened in both urban and rural areas.
More than 80,000 Viet Cong and NVA troops mounted simultaneous assaults on more than 100 cities, towns, and hamlets across the South.
I would guess, however, that your book says C is the right answer. The most visible attacks were those that happened in cities. For example, the attack on the US Embassy in Saigon and the attack on Hue were the most high profile attacks. For this reason, I would guess that your book says C is the right answer, but you really should check to confirm this.