The solution which have lowest freezing point is

The higher the value of the van't Hoff factor (i), the higher will be depression in freezing point and the lower will be the freezing point of solution.

$\text{So, '}i\text{' value,}$ $0.2{\mathrm{MK}}_2{\mathrm{SO}}_4=3$

                         $\begin{array}{rl}0.2\mathrm{MKCl}& =2\\ 0.2\mathrm{M}{\mathrm{NaNO}}_3& =2\\ 0.2\mathrm{M}{\mathrm{MgSO}}_4& =2\end{array}$

$\text{Hence ,}0.2{\mathrm{MK}}_2{\mathrm{SO}}_4 \text{will have lowest freezing point.}$