We need to find the side lengths of the square. We can see that the triangle is a right isosceles, since it has a right angle and the two legs are the same length. Thus, the side ratios are 1:1:√2. Since the sides are 8, we know that the hypotenuse must be 8√2, and therefore so are the sides of the square. If we square the side length we get

8√2x8√2 = 128

so the answer is D.