Which of the following effects is associated with freezing point depression?

Freezing point depression is a colligative property observed when a solute is added to a solvent, resulting in a lower temperature at which the solvent will freeze. The addition of a solute disrupts the orderly arrangement of solvent molecules necessary for solid formation, thus requiring a lower temperature for the phase transition from liquid to solid. This is why the correct choice identifies a decrease in the temperature at which solid formation occurs.

In the context of this question, vapor pressure is not increased due to freezing point depression; in fact, the presence of a solute generally leads to a decrease in vapor pressure compared to that of the pure solvent. The density of the liquid phase could either increase or remain similar, depending on the nature of the solute and its concentration. Boiling point elevation is another phenomenon related to colligative properties but distinct from freezing point depression, as it refers to the increased temperature required to boil a solution compared to the pure solvent. Therefore, identifying the decrease in the temperature at which solid formation occurs accurately describes the freezing point depression effect.