Answer:
A. P
Explanation:
Melting is the process in which a substance turns from solid state into liquid state. This process occurs when enough heat energy is supplied to the solid substance, so that the molecules inside it start to vibrate so faster that they start breaking the bonds that keep them together.
When heat is supplied to a solid substance, the temperature of the substance increases, because the molecules vibrate faster and faster (and the temperature of a substance is directly proportional to the average kinetic energy of the molecules). However, when the substance reaches the melting point, the temperature stops increasing: above this point, in fact, all the heat provided is used to break the bonds between the molecules, so that the change of phase can occur. This corresponds to point P in the graph. Later, when the solid has completely melted into a liquid, the temperature starts increasing again (point Q).
