Question

If action is always equal to reaction , explain why a cart pulled by a horse can be moved???????

Answer 1

because the force exerted by the horse to pull cart is greater than the frictional force which is act against it and also greater than normal force act against it.
That's why cart pulled by a horse can be moved.

Answer 2

    The horse exerts force on the cart and the cart exerts force on the horse.  The cart also experiences a frictional force on it from the ground.  But it is less than the force exerted by the horse on the cart.  So the cart moves forward.

   The horse exerts also a force on the ground. This is the friction force (push) between ground and horse.  This force is more than the force exerted by the cart on the horse.  So the horse moves forwards.