Answer: the best option to answer the question is letter C, "All of these"
Step-by-step explanation:
The question requires us to choose the best option about the plateau (flat) sections of heating curve provided.
The graph provided shows the change in temperature in function of time, considering a solid substance (at the beginning of the curve) that is being heated. When enough heat is provided to this solid, it reaches the temperature where the solid can transform into a liquid - its melting point. At this point, the phase change occurs and the temperature does not increase. On the other hand, the substance needs to absorb energy to "promote" the phase change. This transformation happens at the first plateau shown in the graph.
Increasing the temperature, the liquid now is being heated until it reaches the temperature where it can transform from liquid to gas - the boiling point. Similarly to the previous flat section, the phase transformation (liquid -> gas) will occur at a constant temperature and the susbtance needs to absorb enough energy to change its physical state.
Further increasing the heat, the temperature of the substance in gas form is now increasing.
Considering the explanation above, we can say that at the flat sections of the graph, there is a phase transformation ocurring, the temperature does not increase (it is kept constant during the phase transformation) and the substance is absorbing heat to "promote" the change transformation.
Therefore, the best option to answer the question is letter C, "All of these".