Answer
Answer choice $C$: Positive real numbers
Work Step by Step
Let's use process of elimination. You can eliminate answer choice $D$, because "All real numbers" includes negative numbers, and for a circle, the circumference cannot be negative. You can also eliminate answer choice $A$,
and answer choice $B$, because not only integers can be the radius, but any positive real number, like fractions and decimals, can be the radius.
The domain can only be real numbers that are positive. The answer is $C$
