During freezing of a solution the following are in equilibrium

During depression of freezing point in a solution, liquid solvent and solid solvent are in equilibrium. During freezing of a solution, only the solvent freezes out and the equilibrium exists between solid and liquid forms of the solvent. Vapour pressure of the solid and liquid forms must be same at freezing point, because, otherwise, the system would not be at equilibrium and the lowering of vapour pressure leads to the lowering of temperature at which the vapour pressures of the liquid and frozen forms of the solution will be equal. Hence, option(a) is correct.