D is the correct answer.
There's 2 types of energy existing in a substance, kinetic and potential energy. Kinetic energy, is the movement of the molecules. The higher the kinetic energy is, the faster the molecules move. Kinetic energy can also be expressed with temperature. Meanwhile, potential energy represents the bonds between the molecules. The further 2 molecules are, the higher their potential energy is.
There's always 2 stages when heating, increasing in temperature, and changing of state. Since only kinetic energy is related to the temperature, only kinetic energy is increased when the substance is just heating up. But since changing state requires bonds to become further apart, only the potential energy is increased, kinetic energy is not changed. Thus, we can conclude that during phase change, the temperature of the substance is not increased because only the p.e. is changing.
So back to the graph, in phrase AB, the temperature is rising, and we can conclude that only the k.e. is changing therefore the solid remains solid. The molecules are just moving faster.
In BC, the temperature is remained stable while heat is still supplied. We can conclude that only the p.e. is changing. So in this time, the solid is changing into liquid, the molecules are further apart so the p.e. is increasing.
In CD, the temperature starts increasing again, so it is clear that only the k.e. is changing. Since the substance already turned into liquid and the k.e. is increasing, the molecules of the liquid are sliding faster and faster by time.
Back to the choices given, A and C is definitely not correct because the substance turned into liquid already. B is also not correct because if you still remember, in intermolecular bonds between the molecules are represented by the potential energy, not kinetic energy.
So therefore, at the end, we can conclude that D is the correct answer, "The particles of the liquid slide around faster as the kinetic energy of the particles increases".
