ABC is a triangle right angled at A

Now,

∠A + ∠B + ∠C = 180^o

∠B + ∠C = 90^o

And ∠A is 90^o

Since, "A" is the largest angle of the triangle, the side opposite to it must be the largest

So, BC is the hypotenuse which is the largest side of the right-angled triangle ABC.