Explanation:
The third law of motion, also known as Newton's third law, states that for every action, there is an equal and opposite reaction. This means that whenever an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

Examples of the Third Law of Motion:
a) Walking: When a person walks, they exert a force on the ground in the backward direction. As a result, the ground exerts an equal and opposite force on the person, propelling them forward.

c) Walking on a boat: When a person walks on a boat, they exert a force on the boat in the backward direction. In response, the boat exerts an equal and opposite force on the person, keeping them in place.

d) Cycling: When a person pedals a bicycle, they exert a force on the pedals in the downward direction. As a result, the pedals exert an equal and opposite force on the person's feet, propelling the bicycle forward.

Why Skiing is not an example of the Third Law of Motion?
Skiing involves sliding down a slope on skis. While skiing, the person exerts a force on the skis in the backward direction to maintain balance and control their speed. However, the force exerted by the skis on the person is not equal and opposite. The force exerted by the skis on the person is primarily in the upward direction, providing support and preventing the person from sinking into the snow. Therefore, skiing does not demonstrate the equal and opposite reaction required by the third law of motion.

In Conclusion:
Skiing is not an example of the third law of motion because the force exerted by the skis on the person is primarily in the upward direction, providing support, and not in the equal and opposite direction as required by the third law of motion.