The claim that explains what Stefan must do so that the cart reaches the end of the track is (G) use more force. Adding more books to the cart increases its mass, which requires more force to overcome the inertia and move it forward.
The force that makes the car roll forward in Alonso's case is (C) a push from Alonso's hand. When Alonso stretches the rubber band and releases it, the stored potential energy is converted into kinetic energy, causing the rubber band to push the car forward.