A south pole of a bar magnet is brought near the north pole of another bar magnet placed on rollers (see figure). The second magnet moves toward the first one without the two ever touching.  Which statement about the force involved is correct?

Unlike poles of two magnets attract each other, and this attraction acts even when the magnets are not touching. So the force is a magnetic force and an example of a non-contact force. Rollers only reduce friction so the motion is visible; they do not transmit the force. Friction in this set-up opposes motion, not causes it. Gravitational force between two small magnets is negligibly small compared with the magnetic force.