Which of the following examples represent periodic motion?
(a) A swimmer completing one return trip from one bank of a river to other bank. (b) A freely suspended bar magnet displaced from its `N-S` direction and released. (c) A hydrogen molecule rotating about its centre of mass. (d) An arrow released from a bow.

Explanation

To determine which of the given examples represent periodic motion, we first need to understand what periodic motion is. Periodic motion is defined as the motion of an object that repeats itself at regular intervals of time. In simpler terms, if an object moves from a starting position to a certain point and then returns to its starting position, and this process can be repeated indefinitely in a consistent time frame, it is considered periodic motion.

Now, let's analyze each example one by one:

### Step 1: Analyze Example (a)
**Example (a)**: A swimmer completing one return trip from one bank of a river to the other bank.

- The swimmer moves from one bank to the other and then returns. However, this motion does not repeat indefinitely in a fixed time period. The swimmer may take different amounts of time for each trip based on various factors (like fatigue, speed, etc.).
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