Explain temperature change and process of change on a cooling curve:

When we heat a solid, the energy supplied is used to increase the kinetic energy of its molecules, and thereby its temperature increases. ... From solid to liquid at melting point or from liquid to gas at boiling point) is termed as its latent heat.

Process:

A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid. This is because the matter has more internal energy as a liquid or gas than in the state that it is cooling to.  

The initial point of the graph is the starting temperature of the matter, here noted as the "pouring temperature". When the phase change occurs there is a "thermal arrest", that is the temperature stays constant. This is because the matter has more internal energy as a liquid or gas than in the state that it is cooling to. The amount of energy required for a phase change is known as latent heat. The "cooling rate" is the slope of the cooling curve at any point.

potential energy is directly proportional to height.

more height = more potential energy.

greatest increase in height = greatest increase in potential energy

where are the points with the greatest increase in height ?

has to be c-e or d-e .