Final answer:
The exact ratio representing cos(a) cannot be determined from the provided information, as it requires the adjacent side to the hypotenuse of a right triangle, and the relationship between the given numbers is not provided.
Step-by-step explanation:
The ratio that represents cos(a) is the quotient of the adjacent side of angle 'a' in a right triangle to the hypotenuse of the triangle. Without specific values of a right triangle's sides, we cannot determine the exact ratio for cos(a). However, in trigonometry, cosine is commonly given as part of a right-angled triangle: if 'a' is an angle in a right triangle, then cos(a) = adjacent/hypotenuse. It is important to note that the problem, as presented, does not provide a clear context to choose one of the given options (a-d). Therefore, without additional information about the triangles or the angles to which these values correspond, accurately selecting the ratio that represents cos(a) from the given options is not possible.