The correct formula to calculate the circumference of a circle given the radius is:

C = 2πr

So, the correct answer is C. C = 2πr.

This formula comes from the fact that the circumference of a circle is directly proportional to its diameter, which is twice the radius (2r), and the proportionality constant is π (pi).

Would you like further details or have any questions?

Here are five questions to expand this topic:

1. How would the formula change if you were given the diameter instead of the radius?
2. Can you calculate the circumference of a circle with a radius of 5 cm?
3. What is the value of π to two decimal places?
4. How do you calculate the area of a circle?
5. How does the circumference of a circle compare to its area for the same radius?

Tip: Remember that the diameter of a circle is always twice the radius!