first we need to find the hypotenuse by using a^2 + b^2 = c^2 (where c is the hypotenuse and a and b are the other sides)
8^2 + 5^2 = 64 + 25 = 89, so c = sqrt89.
cosine is adjacent over hypotenuse: 8/sqrt89
rationalize the fraction: (8*sqrt89)/89
the answer is C.