Answer:
C. a cone with a base circumference of about 63 mm.
Step-by-step explanation:
Rotation of a given object about a line is a process of transformation. The simple rule used here is:
         (x, y) →  (-y, x)
When a triangle is rotated about the given line a, this would result to a cone. Thus a 2 dimensional figure is been converted to a 3 dimensional figure.
The cone would have a circular base of radius = 10mm, since each side of the triangle measures 20mm.
The circumference of the circular base of the cone = 2[tex]\pi[/tex]r
                       = 2 × [tex]\frac{22}{7}[/tex]  × 10          (take [tex]\pi[/tex] = [tex]\frac{22}{7}[/tex])
                       = 62.857 mm
The circumference of the base of the cone ≅ 63 mm.