The correct formula to find the circumference of a circle when the radius is given is:
\[ C = 2\pi r \]
However, since this option is not presented in your list, the closest correct expression is:
C = πd, where d is the diameter (which is equal to 2r).
None of the responses you provided correctly represent the standard formula for circumference (C = 2πr), but if you're looking for a formula related to the radius, C = π2r is still not correct. It may be a misunderstanding in provided responses.
So, just to clarify: the formula for circumference is C = 2πr. If you're limited to select from the responses given, consider discussing the proper formulas with whoever provided those options for clarification.