Which of the following aqueous solutions of equal molality has the HIGHEST vapour pressure at a given temperature (assume complete dissociation of electrolytes)?

At equal molality the lowering of vapour pressure rises with the number of solute particles. Glucose gives 1 particle per formula unit, NaCl gives 2, $\mathrm{K}{}_2\mathrm{SO}{}_4$ gives 3 and $\mathrm{AlCl}{}_3$ gives 4. Fewest particles means smallest lowering, hence the highest vapour pressure for glucose.