Answer:
Horse will become slower as we add more number of bags on the trolley
Explanation:
If a horse is pulling the cart then here horse must have to apply sufficient force on the cart so that the applied force is just counterbalancing the friction force on cart.
Now here we know that friction force is given as

where
m = total mass of the cart
Now if we assume here that horse will is giving constant power to the cart
so we will have

Power is given as product of applied force by horse on cart and its velocity
now as we know that if we put more bags on the cart then it will increase the weight of cart.
Due to increase in weight of cart there will be more friction force on it so horse must have to apply more force to counterbalance the friction force.
Now in order to maintain constant power velocity of horse must have decrease.
So horse will become slower as we add more number of bags on the trolley