I know is between a and C can someone please help me

For a regular polygon, the rotation about its center that keeps the polygon unchanged is:
[tex]\frac{360}{n}\text{ degrees rotation}[/tex]where n is the number of sides of the polygon.
In this case, n=6, therefore, a 360/6=60° clockwise rotation about the center of the hexagon would result ina hexagon with the same vertices.