Concept used: depression in freezing point

For a solution containing a non-volatile solute:

• Vapour pressure of the liquid solution is lower than that of the pure liquid solvent at every temperature.
• The solid solvent vapour pressure curve remains unchanged, because the solute is not present in the solid solvent crystal.

Condition for freezing point

The freezing point is the temperature at which the vapour pressure of:

• solid solvent
• liquid phase

become equal.

So:

• For the pure solvent, freezing point is where the solid solvent VP curve intersects the pure liquid solvent VP curve.
• For the solution, freezing point is where the same solid solvent VP curve intersects the lower liquid solution VP curve.

Why freezing point decreases

Since the solution has lower vapour pressure than the pure solvent, the intersection with the solid solvent curve occurs at a lower temperature.

Therefore,

and graphically the solution freezing point must lie to the left of the pure solvent freezing point on the temperature axis.

What the correct graph must show

The correct vapour pressure vs temperature graph must satisfy all of these:

1. Two liquid curves: pure solvent above, solution below.
2. One solid solvent curve, unchanged.
3. Intersection of solid curve with pure liquid at higher temperature.
4. Intersection of solid curve with solution liquid at lower temperature.
5. The horizontal difference between these temperatures is labeled $\Delta T_f$.

Matching with options

Among the given choices, the graph that shows exactly this arrangement is Option C.

Conclusion

Hence, the correct graphical representation of depression in freezing point is: