Consider a right triangle ABC, right-angled at B. Then,
AC
In a triangle, the side opposite to the bigger angle is larger than that opposite to the smaller angle. In the above question, it is given that the triangle is a right angled triangle. So, the sum of the other two angles will be 90∘. So, the longest side would be the one opposite to the right angle, i.e. AC. Hence, AC is the longest side.