D is the correct answer
(in a right triangle, if the angles are 90,45,45,the legs are equal)
[tex] {8}^{2} + {8}^{2} = x \\ 6 4 + 64 = x \\ 128 = x[/tex]
[tex]x = \sqrt{128} = \sqrt{2 {}^{7} } = \sqrt{2 {}^{6} \times 2} \\ = 8 \sqrt{2} [/tex]
good luck