Answer
Direct variation.
Work Step by Step
A direct variation between two variables x and y can always be written as: $y=kx$ ($k\ne0$), in which $k$ is a constant.
Because $2\pi$ is a constant (always equal to about $6.28$) and the relationship between C (circumference) and r (radius) is $C=2\pi r$, they are directly related to each other.
