a. Forces acting on the pulley A:

Looking into the fig, we can say the boy is pulling the string in downwards direction using pulley A.

Hence two forces come to action in this case-

1. Electromagnetic force between the string and pulley A due to tension.

2. And the Earth and pulley system has a pair of gravitational force between them.

b.

Now, when the boy pulls the string downwards, the following forces are experienced by him-

1.Electromagnetic force between the string and boy due to tension.

2.And a pair of gravitational force between the Earth and the boy.

c.

Now, the block C is moving in upward direction, hence the following forces are exerted on the block-

1. Electromagnetic force between the string and block C due to tension.

2. A pair of gravitational force between block C and the Earth.