Answer:
Let's explain this briefly.
Suppose that we have a piece of ice (this is, solid water) now we give energy to the piece of ice, so the temperature of the ice increases. There is a point where the piece of ice will start a change of phase, at this point the temperature of the ice stops increasing because all the energy we give to the ice is used in the change of phase.
Once we have a complete change of phase, the temperature can increase again, and now we will have liquid water.
If we keep increasing the temperature we will see this happen again, when we have the transition from liquid to gas.
(and a similar thing happen when we have a material in a given phase and we remove heat from the material).
In the images we can see the different changes of phase of water.
1) In the first image we can see the circle in a part where the temperature is constant, so the temperature does not change in this part, which means that there is a change of phase happening.
2) Here we have the circle in a diagonal line, so here the temperature is changing, meaning that we have an increase of temperature in this region.
3) Here we want to know what the x-axis represents, this should rerpesent the energy that is being given to the material (so in some parts we see that the temperature increases and in other parts we see that the material changes of phase)
Then here the correct option is heat over time.
4) The freezing point is the temperature in which the change of phase from liquid to solid happens (or solid to liquid).
In the graph we can see that this change of phase happens at the temperature T = -210°C
Then the correct option is -210°C (The last option)
