In a pulley system with a cart and a hanging mass, there are two action-reaction forces: between the cart and the hanging mass, and between the cart and the pulley.
In a pulley system where a cart with mass M is being pulled by a hanging mass m, there are two action-reaction forces at play.
The first action-reaction pair is between the cart and the hanging mass. The cart exerts a force on the hanging mass, and the hanging mass exerts an equal and opposite force on the cart. This force allows the cart to move.
The second action-reaction pair is between the cart and the pulley. As the cart moves, it exerts a force on the pulley, and the pulley exerts an equal and opposite force on the cart. This force helps to change the direction of motion of the cart.