As $\mathrm{\Delta }{\mathrm{T}}_{\mathrm{f}}\propto \mathrm{m}\times \mathrm{i}$
Urea and glucose do not dissociate in aqueous medium, hence their solution has lowest depression in freezing point as for them i is only 1. It will be maximum for 0.1 M $\mathrm{A}{\mathrm{l}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3$ and 0.1 M $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4$ as $\mathrm{m}\times \mathrm{i}$ will be maximum for them.
$\text{Δ}{\text{T}}_{\text{f}}\propto \text{i}\times \text{m}$
${\text{T}}_{\text{f}}\propto \frac{1}{\text{i}\cdot \text{m}}$
$\therefore \text{Δ}{\text{T}}_{\text{f}}=0={\text{T}}_{\text{f}}$