When $H{g}_{\mathit{2}}{l}_{\mathit{2}}\mathit{\left(}\mathit{S}\mathit{\right)}$ is added to a solution of $\mathrm{KI}\left(\mathrm{aq}\right)$  then

$\text{2KI}+{\text{Hgl}}_2\to {\mathrm{K}}_2{\mathrm{Hgl}}_4$
2KI will give 4 particles whereas ${\mathrm{K}}_2{\mathrm{Hgl}}_4$ will give $\{{\text{K}}_{\text{2}}{\text{Hgl}}_4\rightleftharpoons 2{\mathrm{K}}^{+}+{\mathrm{Hgl}}_4^{2-}\}$ 3 particles. 
As number of particles is decreasing.
$\therefore$The colligative property will decrease.