a) False. This can be proven by the fact that a rotation around a point (not necessarily on the circles) can map 2 circles onto each other.
b) False. Rotations, reflections, and dilations (the last one applies if the circles have different radii) can map one circle onto another.
c) True. All circles are a uniform distance (the radius) from a point (the center), so there is only one length that defines a circle, the radius. Since a:b = a:b, where a = the radius of one circle while b = radius of another circle, that means that 2 circles will always be similar (because all corresponding lengths are in the same ratio)
Hope that helped!
