Question

Find the length of side c of a triangle if side a = 15 and side b = 20. A. 25 B. 17.5 C. 20 D. 15

Answer 1

If one side of a triangle is 15 and another side is 20, then the third side
can be any length between 5 and 35. There are an infinite number of
possible triangles.

IF it happens to be a RIGHT triangle AND side 'a' and side 'b' happen
to be its legs, then the length of the third side is 25.  But you didn't
mention any of those details.