Periodic Motion:
Periodic motion refers to a type of motion that repeats itself after a certain interval of time. It is characterized by the regular and repetitive back-and-forth or oscillatory movement of an object around a fixed point or along a fixed path. The time taken to complete one full cycle of motion is called the period.

Examples of Periodic Motion:
Among the given options, all of them demonstrate periodic motion. Let's discuss each example in detail:

1. Motion of Needle of a Sewing Machine:
The motion of the needle of a sewing machine exhibits periodic motion. The needle moves up and down in a regular pattern as it stitches through the fabric. It completes one full cycle of motion when it moves from its highest point to its lowest point and back up again. This motion repeats itself as long as the sewing machine is in operation.

2. Motion of the String of a Guitar:
The motion of the string of a guitar also displays periodic motion. When a string is plucked or strummed, it vibrates back and forth. This vibration creates sound waves that produce musical notes. The string oscillates with a specific frequency, and the motion repeats itself as long as the string continues to vibrate.

3. Pendulum of a Clock:
The pendulum of a clock is a classic example of periodic motion. It swings back and forth under the influence of gravity. The pendulum moves from one extreme position (highest point) to the other extreme position (lowest point) and then back again. This motion repeats itself with a fixed time period, which determines the ticking of the clock.

Conclusion:
In conclusion, all of the given options (a) Motion of needle of a sewing machine, (b) Motion of the string of a guitar, and (c) Pendulum of a clock demonstrate periodic motion. These examples involve the regular and repetitive movement of objects around fixed points or along fixed paths, with each motion cycle completing in a specific time period.

It is d) All of These