The diagonal of a square can be found using the Pythagorean theorem, which states that in a right triangle, the square of the length of the hypotenuse (diagonal) is equal to the sum of the squares of the lengths of the other two sides. In this case, the square has sides of length 8, so the length of each side is 8. Using the Pythagorean theorem, we can calculate the length of the diagonal as the square root of (8^2 + 8^2) = √(64 + 64) = √128 = 8√2. Therefore, the correct answer is C.