Which of the following solution has the highest freezing point?

Freezing point will be given by formula $\Delta T_f=i{K}_{f}m$

(a) 0.1 mol $KCl$ in 1 kg water $KCl\to K^{+}+C{l}^{-}$  Thus, $i=2$

(b) 0.1 mol $K_{2}S{O}_4$ in 1 kg water $N{a}_{2}S{O}_4\to 2N{a}^{+}+S{O}_4^{2-}$ Thus, $i=3$

(c) 0.1 mol urea in 1 kg water urea is a non-electrolyte Thus, $i=1$

$\Delta T_f=i\times K_f\times m=I\times K_f\times 0.1=0.1K_f$

(d) 30g glucose in 1 kg water $\Delta T_f=i\times K_f\times m$

Glucose is non-electrolyte $i=1$

$\Delta T_f=i\times K_f\times \frac{30/180}{1}=0.166K_f$

The highest freezing point will be of the solution having the lowest $\Delta T_f$ value.  Thus, the highest freezing pointy of option (c) because it, give lowest $\Delta T_f$ value.