In the depression of freezing point experiment
A. Vapour pressure of the solution is less than that of pure solvent
B. Vapour pressure of the solution is more than that of pure solvent
C. Only solute molecules solidify at the freezing point
D. Only solvent molecules solidify at the freezing point

The depression of freezing point is a colligative property that appears when a non-volatile solute is dissolved in a volatile solvent.

Case 1 : Statement A
Statement A says: “Vapour pressure of the solution is less than that of pure solvent.”
According to Raoult’s law, addition of a non-volatile solute lowers the vapour pressure of the solvent:
$$P_\text{solution}=x_\text{solvent}\,P^\circ_\text{solvent}$$
Since $$x_\text{solvent}\lt1$$, $$P_\text{solution}\lt P^\circ_\text{solvent}$$. Hence Statement A is correct.

Case 2 : Statement B
Statement B is the opposite of Statement A, therefore it is false.

Case 3 : Statement C
Statement C says: “Only solute molecules solidify at the freezing point.”
At the freezing point an equilibrium exists between solid solvent and liquid solution. The solid phase that separates out is pure solvent; solute molecules remain in the liquid phase. Hence Statement C is false.

Case 4 : Statement D
Statement D says: “Only solvent molecules solidify at the freezing point.”
As explained above, the solid that forms is pure solvent. Hence Statement D is correct.

Thus, the correct statements are A and D only.

Option A (A and D only) is the correct answer.