We see in the figure different points where the student stops that are at different heights. First, you must take into account that the less height there is, that is, for example, point A, the greater the atmospheric pressure. This happens because there will be more layers of air above the earth's surface. On the contrary, if the student is at a higher point, for example, point D, there will be fewer layers of air above him, and the atmospheric pressure will be lower.
Now, atmospheric pressure is very important because it directly affects the boiling temperature. Remember that the boiling point is the temperature at which a liquid passes into the gas phase.
In the gas phase, the molecules are much further apart than in the liquid phase and can move more freely. The pressure exerts a force on these molecules, when the pressure increases it will be more difficult for the molecules to move freely, so they will require more energy to do so. Higher energy equals higher temperature, in this case, boiling temperature.
Therefore, at point A the atmospheric pressure will be higher and the boiling temperature must also be higher. At point D we will have the lowest atmospheric pressure, the molecules will be able to move more easily and the boiling temperature will be lower.
In conclusion, the water has the lowest boiling point at point D. So the answer will be C. D