Final answer:
The decrease in melting temperature due to solute concentration is associated with Raoult's law, but it's the van't Hoff factor that is used in calculations for freezing point depression and boiling point elevation.
Step-by-step explanation:
The decrease in melting temperature (or freezing point depression) as a function of solute concentration is described by the colligative property relating to the van't Hoff factor (i). The correct answer to your question is: b) Raoult's law as it involves the decrease in vapor pressure which indirectly affects the melting temperature. However, the van't Hoff factor is used in the calculation of freezing point depression and boiling point elevation, not directly for melting temperature. The freezing point depression (ΔTf) equation is ΔTf = Kf * m * i, where Kf is the freezing point depression constant for the solvent, m is the molality of the solution, and i is the van't Hoff factor, which accounts for the number of particles the solute dissociates into in solution. It is important to note that the van't Hoff factor decreases as solute concentration increases, indicating a deviation from ideal solution behavior.