Option (iv) is correct because during a collision, the two carts exert equal and opposite forces on each other. This is based on Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. The difference in mass between the loaded cart and the empty cart affects their individual accelerations (as a larger mass will have a smaller acceleration for the same force), but the magnitude of the force they exert on each other during the interaction must be exactly equal.