The motion of the swimmer is to and fro but the period is not fixed. Hence the motion is not periodic.
The motion of the suspended bar magnet is periodic as it will oscillate about its origin in a certain period.
The motion of the hydrogen molecule about the centre of the mass is the periodic motion as it comes back to its original position in the equal interval of time.
The arrow released moves in forward direction only and does not come back. Hence this is not a periodic motion.