Action and Reaction
According to Newton's third law of motion, every action has an equal and opposite reaction. This means that if an object exerts a force on another object, the second object will exert an equal force in the opposite direction on the first object.

Force of Horse
When a horse pulls a cart, it exerts a force on the cart in the forward direction. This force is the action force. The horse's hooves push against the ground, which creates a force that propels the horse forward. As the horse moves forward, it pulls the cart along with it.

Force of Cart
The cart exerts a force on the horse in the opposite direction. This force is the reaction force. The force of the cart pushing back on the horse is what allows the horse to move forward.

Friction
However, if the ground were perfectly smooth, the horse would have a hard time pulling the cart. The reason for this is friction. Friction is the force that opposes motion between two surfaces that are in contact. When the horse pulls the cart, there is friction between the wheels of the cart and the ground. This friction creates a reaction force that helps the horse move the cart forward.

Conclusion
In conclusion, the horse is able to pull the cart because of the action-reaction principle. The horse exerts a force on the cart, and the cart exerts an equal and opposite force on the horse. The friction between the wheels of the cart and the ground also helps to create a reaction force that allows the horse to move the cart forward.

Due to frictional force ..