To rearrange the formula for the circumference of a circle, \(C = 2\pi r\), in order to find the radius \(r\), you would isolate \(r\) by dividing both sides of the equation by \(2\pi\):
\[ r = \frac{C}{2\pi} \]
So the correct response is:
r = Start Fraction upper C over 2 pi End Fraction.