According to the provided context, periodic motion is defined as 'a motion that repeats itself at regular intervals of time.' The context further clarifies that 'Every oscillatory motion is periodic' and 'Circular motion is a periodic motion, but it is not oscillatory.' Option (b) represents oscillatory motion because when a freely suspended bar magnet is displaced from its equilibrium (N-S direction) and released, it oscillates back and forth about the equilibrium position, making it periodic. Option (c) represents rotational motion where the hydrogen molecule rotates about its centre of mass, repeating its motion at regular intervals, which is analogous to circular motion and hence periodic. Option (a) is NOT periodic because it describes a single return trip with no repetition. Option (d) is NOT periodic as the arrow moves in one direction without repeating its motion.