In the first case, there is a pendulum which represents periodic motion. In the second case, there is a guitar which has vibratory motion involved in it. The second case also represents periodic motion as a person playing guitar needs to the follow rhythm. The third case shows a bell which is usally used to proclaim the particular hour, which also repeats itself after fixed intervals of time. Hence it is periodic motion. In the last case, it is again a musical instrument which needs rhythm to be played properly. Hence the answer should be C which is "periodic motion".