I get why the value of x can be 90, but why can it not be a value other than 90, say 100? According to the book, the answer is C.
This is a quantitative comparison with A: x and B: 90
This is due to the Pythagorean Theorem. If a^2+b^2=c^2 where a,b are legs and c is the hypothenuse, then the triangle in question is a right triangle. It also works in the opposite direction. If the triangle is a right triangle, then a^2+b^2=c^2.
I don’t know if what your math backgroud is, but this theorem is of the iff form i.e. if and only if, or a bidirectional theorem