By rotating the triangle about line a, the shape formed is a cone. The circumference of the base would be found by taking the length of the tiangle to be the diameter.
Recall, circumference = 2 x pi x r
where r is the radius
diameter = 2 x radius
20 = 2 x radius
radius = 20/2 = 10
pi = 3.14
Circumference = 2 x 3.14 x 10
Circumference = 62.8
Approximately,
Circumference = 63
Thus, option C is correct