Answer:
The correct answer is - They gently shake the pan causing the marbles to move back and forth.
Explanation:
When water is heated the molecules present in its liquid state start to move and vibrate faster and allows the water to expand and increase in volume. If the heat is continuously applied to the water its molecules move even faster and escape in the form of molecules of vapor to the atmosphere.
To exhibit this phenomenon by the marble and pan, Richard and Brooke should gently shake the pan causing the marbles to move back and forth which shows faster vibration and movement of molecules.