Final answer:
Water particles become fixed in place at the freezing point, which is 0°C. This is where the solid and liquid states are in equilibrium, and it is the same as the melting point for the substance.
Step-by-step explanation:
As water is cooled, its particles become fixed in place at the freezing point. This is the temperature at which a liquid changes phase and becomes a solid. For water, this transition, wherein the water particles become fixed in place, occurs at 0°C. The Celsius scale uses the freezing point of water as one of its reference points, making it a critical concept in understanding phase transitions.
It is important to note that the melting point of a solid is the same as the freezing point of the liquid. At this temperature, the solid and liquid states are in equilibrium, which is represented by the equation H₂O (s) = H₂O (l). Therefore, the correct answer to the student's question is a) Freezing point.