(1 ~mol) each of the following compounds is dissolved in (1 ~L) of...

Bond dissociation energy increases in the order. ($\mathrm{HI} \lt \mathrm{HBr} \lt \mathrm{HCl} \lt \mathrm{HF}$) ($\mathrm{HF}$) is most stable halogen acid and ($\mathrm{HI}$) is the least. Higher the bond dissociation energy, lower is the degree of ionisation. As we know ($\Delta$T_b=i K_b m). Hence ($\Delta$T_b) value is largest in HI. All colligative properties depend on number of particles.