Where were the principal strikes of the Tet Offensive?A) Khe Sanh B) North Vietnam C) urban areas D) rural areas

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.

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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.