Ithink it's reflection but i'm not so sure..

You are right, it's a reflection and the axis of this reflection is the perpendicular bisector of CC'

This is, indeed, reflection because the figure is mirrored across an imaginary line. Although a 180° rotation would make the same shape, the "B" and "A" would be swapped. It is not a dilation because this is the process in which a shape grows or shrinks. Finally, it is not a translation because the figure was not simply moved. 

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.