Final answer:
During its upward movement, a ball experiences an unbalanced force due to gravity acting downwards, which slows the ball down until it reaches the peak of its trajectory. So the correct option is c.
Step-by-step explanation:
When a ball is thrown vertically upwards and is moving towards its peak, it experiences an unbalanced force that is directed downwards. This unbalanced force is due to gravity, always acting on the ball to pull it towards the Earth. As the ball moves upwards, its speed decreases because the force of gravity is acting opposite to its direction of motion. Eventually, at the peak of its trajectory, the ball's velocity will be zero before it starts descending again. Therefore, the correct option is c. the ball is experiencing an unbalanced force, it is directed downwards.