Which from following solutions in water on complete dissociation exhibits minimum freezing point depression?

The depression in freezing point is given by $\Delta T_f = i \times K_f \times m$.

For complete dissociation, the van't Hoff factor $i$ is equal to the number of ions produced per formula unit.

For $0.2$ m Potassium chloride (KCl): $i = 2$, $i \times m = 2 \times 0.2 = 0.4$

For $0.1$ m Sodium chloride (NaCl): $i = 2$, $i \times m = 2 \times 0.1 = 0.2$

For $0.05$ m Aluminium phosphate (AlPO$_4$): $i = 2$, $i \times m = 2 \times 0.05 = 0.1$

For $0.15$ m Magnesium sulphate (MgSO$_4$): $i = 2$, $i \times m = 2 \times 0.15 = 0.3$

Since $0.05$ m Aluminium phosphate has the minimum value of $i \times m$, it exhibits the minimum freezing point depression.

Answer: $0.05$ m Aluminium phosphate