Is is actually quite easy if you think about it! The formula for finding the area of a circle is:
Area= pi x r2
Or in this case, Pi (3.14) x the radius squared.
In the picture above, the radius (Half of the diameter) is 3 units.
3x3 = 9
Now all that is left is to multiply it by pi!
9 x 3.14 = 28.26
So the answer is 28.26, or C! Hope this helped :)!