Ok, so the 4pi is indicating toward the arc length. Since the radius of the circle is 18, the circumference of the circle is 36pi. If you find the ratio of the arc length to the circumference, you can find the ratio of the indicated angle to the total number of degrees in a circle.
4pi/36pi = 1/9
(indicated angle)/360 = 1/9
(indicated angle = 40
The answer is the 4th choice, or 40 degrees.