A ladybg in the bottom of a whirling tin can feels a centrifugal force pushing it against the bottom of the can. Is there an outside source of this force? Can you identify this as the action force of an action-reaction pair? If so, what is the reaction force?

The reality of Centrifugal Force is clear in a rotating frame of reference. You are sitting on an amusement park ride that is a seat rotating on the end of a set of chains. The Frame of Reference is the ride itself and it rotates with you and your seat. Relative to that Frame of Reference you are in equilibrium and motionless. If you are in equilibrium then the Sum of the Forces acting on you must equal zero. The seat is exerting a force on your but (Centripetal Force), By Newton’s 3rd Law, if you are motionless then, your but must exert an equal and opposite force on the seat