$\because$ Freezing point of solution (T_f) = ${\mathrm{T}}_{\mathrm{f}}^0-{\mathrm{\Delta T}}_{\mathrm{f}}$

so for maximum Tf ⇒ $∆$T_f must be minimum

$\because$ $∆$T_f = K_f .m.i

where   K_f → Constant

             m →     Molality (same for all)

so for minimum $∆$T_f ⇒ i should be minimum which is for [Co(H₂O)3Cl₃].3H₂O