The correct option is D No forward force is being applied by the horse or the cart
The horse tied to cart is standing stationary on road denotes that the net force acting on the horse as well as the cart individually is zero. If it is standing stationary, it means that horse is applying force on the road perpendicularly and road is also applying same magnitude of force in the opposite direction, which makes the net force on horse to be zero and hence no forward force on the horse is there.