The correct option is A 0.5M Al2(SO4)3
Vapour pressure depends on the concentration of non-volatile solute.
For Al2(SO4)3, we need to consider C×i, where C=Concentration
Hence, for Al2(SO4)3 , C×i=0.5×5=2.5
For Urea solution, C=12×100060×100=2M
For NaCl solution, C×i=1×2=2
For Glucose solution, C=18×1000180×100=1M
Here, concentration of non-volatile solute is maximum for Al2(SO4)3. Hence, it will have lowest vapour pressure.