What is the constant of proportionality for the ratio of the length of an arc and the radius?

A Circumference

B Length of radius

C Length of subtended arc

D Radian measure

Once in a while, you get a question where the answer satisfies the question.

It is not A. The arc measure might be part of the circumference but it is not all of it.

B is included in what you are given.

C and the arc measure are the same thing.

D is your answer. The formula is theta*r where theta is the radian measure of the central angle.