Which of the following 0.10 m aqueous solutions will have the lowest freezing point ? 

Depression in F.P. $\propto$ No. of particles. 
${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3$provides five ions on ionisation 
${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3⟶2{\mathrm{Al}}^{3+}+3{\mathrm{SO}}_4^{2-}$

while KCl provides two ions 
$\mathrm{KCl}\rightleftharpoons {\mathrm{K}}^{+}+{\mathrm{Cl}}^{-}$
${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\text{ and }{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}$are not ionised so they have single particle. 
Hence, ${\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3$have maximum value of depression in FP or lowest F.P