(a) It is not a periodic motion as the swimmer completes only one trip but if he makes more than one trip and time for each trip is same, the motion can be categorised as periodic.
(b) It is a periodic motion because a freely suspended bar magnet if once displaced from N-S direction and released, it oscillates about this position. Hence it is simple harmonic motion also.
(c) It is also a periodic motion.
(d) It is not a periodic motion.