Final answer:
To represent the substance in the liquid phase, the student needs to decrease the average distance between particles.
Step-by-step explanation:
In order to represent the same substance in the liquid phase instead of gas phase, the student must make the following change: C) Decrease the average distance between particles.
In the liquid phase, the particles are closer together compared to the gas phase. This is because the intermolecular forces between the particles in the liquid phase are stronger, causing them to be more closely packed. The particles in the gas phase, on the other hand, are far apart due to the weak intermolecular forces and high kinetic energy.