Which of the following observer is/are non inertial ? 

1. A child revolving in the merry go round
2. A driver in a train moving with constant velocity v
3. A passenger in a bus which is slowing down to a stop

a and c

Since centripetal acceleration is present in merry round it is non inertial frame of reference

Since bus is slowing down to a stop, it means it is experiencing a retardation, so it is non inertial frame of reference

Hence choice (a) and (c) are correct