Freezing point depression and bp elevation is based on the number of particles in the solution. What you need to remember is that ionic compounds (usually, I highly doubt they're going to throw in a non soluble ionic compound) dissolve completely. So the "equivalent" molarity (not exactly sure what the scientific term is) the molarity x the number of soluble particles. For glucose, there's one soluble particle since it doesn't dissociate. For Na2So4 they're three (two Na and one so4) and for (NH4)3po4 they're four (three NH4 and one PO4. Now you need to multiply these integers by the molarity. So glucose's equivalent molarity is 3, na2so4 is 6, and (nh4)3po4 is 4. Thus, the correct answer is na2so4 since highest equivalent molarity means highest values for fp depression and bp elevation.
