ΔTf = i*Kf*m

ΔTf is the depression in freezing point

ΔTf will be maximum when i*m is maximum

so, lowest freezing point if for that solution which has higher value of i*m

0.010 m Li2SO4: i*m = 3*0.010= 0.030

0.015 m MgCl3: i*m = 3*0.015 = 0.045

0.035 m CH3CH2CH2OH: i*m = 1*0.035 = 0.035

0.010 m NaCl: i*m = 2*0.010 = 0.020

Answer: 0.015 m MgCl2