Which of the following aqueous solutions exhibits maximum freezing...

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

Since $K_f$ is constant for aqueous solutions, $\Delta T_f \propto i \times m$.

For $0.1$ m $\text{Li}_2\text{SO}_4$, $i = 3$, $i \times m = 3 \times 0.1 = 0.3$

For $0.5$ m KCl, $i = 2$, $i \times m = 2 \times 0.5 = 1.0$

For $0.05$ m $\text{Al}_2(\text{SO}_4)_3$, $i = 5$, $i \times m = 5 \times 0.05 = 0.25$

For $0.2$ m $\text{BaCl}_2$, $i = 3$, $i \times m = 3 \times 0.2 = 0.6$

The maximum value of $i \times m$ is $1.0$ for $0.5$ m KCl, which will exhibit the maximum freezing point depression.

Answer: $0.5$ m KCl.