Since each side of the square consists of two triangle legs, the side length is 2 * 3√2 = 6√2. The correct answer is D. 6√2.
The diagonal of the square divides it into two 45-45-90 triangles. Each triangle's leg measures half the diagonal, which is 3√2.
The length of side s of the square is 3√2.
The square is divided into two 45-45-90 triangles by the diagonal. The hypotenuse of each triangle is 6, so the legs of each triangle are 6/√2=3√2.
Therefore, the side length of the square is 2*3√2=6√2.
So the answer is D. 6√2.