Answer: first statement, "At segment T-U, the substance changes from liquid to gas and does not change temperature"
Justification:
1) Being it a heating curve, the temperature increases as you move to the right in the time axis.
2) Regarding the first statement: "At segment T-U, the substance changes from liquid to gas and does not change temperature"
It may be true: if the segment T-U is flat (not inclined), means that the temperature is not changing. In this case, the heat provided to the substance is being used to overcome the intermolecucular atraction forces, which permits the particles to get more separated and, in such way, the liquid pass to gas phase.
3) Regarding the third statement: "At segment T -U, the substance changes from a gas to a liquid and changes temperature", it cannot be true.
When a substance changes from gas to liquid, the substance realeses heat and the temperature does not change.
This is the same for any change of phase: boiling, condenstation, melting, freezing.
4) Regarding the third statement: "At segment Q-R, the substance releases heat and increases temperature", it cannot be true.
If the substance releases heat has two only possibilities: keep the temperature (while changing phase) or increasing temperature.
5) Regarding the fourth statement: "At segment Q-R, the substance absorbs heat and decreases in themperature:, it cannot be true either.
If the substance abosrbs heat has two only possibilities: keep the temperature (in case of a change of phase) or decrease temperature.
6) In conclusion, the only feasible statment is the first one.
