Don’t know how to do it.

Answer : C

To solve any question asking if a triangle is acute, obtuse, or right based on side lengths you have to follow this method.
Square all sides of the triangle. Then add the two shortest sides together. If the two shortest sides are bigger than the largest side then the triangle is acute. If the sum is equal then the triangle is a right triangle. And if the sum is less than the longest side then it’s obtuse. Hope this helps :).