Question

A book is sitting on a desk. What best describes the normal force acting on the book?

Answer 1

My best guess would be:

"A force equal in magnitude but opposite in direction"

However I assume that this question is multiple choice, by the way it is introduced. Therefore it would be helpful if these options were also displayed - hence take this as my best guess only.

Answer 2

The normal force on the book ... the upward force of the table against the
book ... must be exactly equal to the weight of the book, and acting in the
direction exactly opposite to the force of gravity.

If the normal force didn't exactly balance the force of gravity on the book,
so that their sum is zero, then instead of just laying there, the book would
be accelerating.