Which of the following aqueous solution has highest freezing point?

(assume complete ionization)

ΔT_f=iK_fm
As m = 0.1 molal for all given solutions, thus, lower the value of i, lower will be the depression in freezing point (ΔT_f) and higher will be the freezing point of the solution.
For Al₂(SO₄)₃, i = 5
For BaCl₂, i = 3
For AICI₃ , i = 4
For NH₄Cl , i = 2
Thus, 0.1 molal NH₄Cl will have highest freezing point.