the answer is A. 40.8
since the problem gave you the diameter all you have to do is multiply the diameter by pi (3.14) to get the circumference.
Extra Help:
If a similar problem gives you only the radius (half the diameter) then you multiply the radius by 2 to give you the full diameter. Once you get that answer you just need to multiply the diameter by pi (3.14) to get the circumference.