14m/s is correct for their velocity.
The key point is that in part b, there is no friction.
It is possible for the actual (assuming they mean actual as in, if there was friction) speed to be 5m/s
If there is just enough friction going down to slow them down to that point. If they were going faster than 14m/s, then it is possible that they were given an external push.
So again, if by "actual" in part c they mean that friction and/or external pushes apply, it is not necessarily a violation of the law of conservation of energy.
The energy would have left as heat and sound due to friction, or extra energy could have been added by an "external push".