Which of the following solution will have highest freezing point? 

Depression in freezing point $\propto$ molality $\times \mathrm{n}$

For NaCl, ${\mathrm{\Delta T}}_{\mathrm{f}}\propto 2\times 2$

For ${\mathrm{AlCl}}_3,{\mathrm{\Delta T}}_{\mathrm{f}}\propto 4\times 1.5$

For ${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3{\mathrm{\Delta T}}_{\mathrm{f}}\propto 5\times 1$

For Urea ${\mathrm{\Delta T}}_{\mathrm{f}}\propto 3\times 1$

Since, lowest depression is observed for solution containing urea, hence it will have highest freezing point.

${\text{∆T}}_{\mathrm{f} }\mathrm{\alpha }\frac{1}{\mathrm{Freezing} \mathrm{point} \mathrm{of} \mathrm{solution}}$