The correct answer is:
As the radius of a circle increases, circumference of the circle increases.
Step-by-step explanation:
The way the statement is written, the radius of the circle is independent and the circumference of the circle is dependent. This makes the slope the change in circumference per change in radius.
If the independent and dependent variables are switched, then the circumference of the circle will be independent and the radius of the circle will be dependent. This makes the slope the change in radius per change in circumference.
Both of these have the same interpretation.