Hi. The equation for circumference is pi x diameter. I imagine this is an equation you need to know. I like to think of it as how pi is circumference/diameter, no matter what, for every circle ever, which is pretty cool. If you rearrange that you get pi x diameter.
Anyways.
Pi is approximately 3.14, which is also something I imagine you’ll need to know.
You have the radius given to you there and the diameter is double the radius, the whole width of the circle. As your radius is 10cm, your diameter will be 20cm.
20 x pi will give you roughly 62.8, so your answer is B. Hope this helps :)