Sugar would not increase the boiling point of the water as much as the others.To show this, we recall the equation for boiling point elevation ∆t:
∆t = i Kb m
The difference in these four solutions will be in the product of the van't Hoff Factor i and molality m: i * m.
If we assume that we have 0.1m of each solute, the products i*m for these solutions are:
Solution Identity of particles i i * m
0.1 m NaCl Na+, Cl- 2 2 * 0.1 = 0.2
0.1 m MgSO4 Mg^2+, SO4^2- 2 2 * 0.1 = 0.2
0.1 m K2SO4 two K+, SO4^2- 3 3 * 0.1 = 0.3
0.1 m C12H22O11 covalently bonded molecules 1 1 * 0.1 = 0.1
Therefore, having the lowest i * m, sucrose will have the lowest boiling point elevation.