We can use the Pythagorean Theorem to find the diagonal length of the suitcase. Let the diagonal length be represented by d. Then,
d^2 = 24^2 + 10^2
d^2 = 576 + 100
d^2 = 676
d = sqrt(676)
d = 26
Therefore, the diagonal length of the suitcase is 26 inches. So the answer is option D.