Raising the boiling point is a colligative property, and as such it depends on the concentration AND on the number of particles in solution. So, find the compound that produces the highest concentration with the most particles in solution, and that will be the answer.
(A) 0.20 M CaCl2 => 3 particles x 0.2 M each = 0.6
(B) 0.25 M Na2SO4 => 3 particles x 0.25 M each = 0.75
(C) 0.30 M NaCl => 2 particles x 0.3 M each = 0.6
(D) 0.30 M KBr => 2 particles x 0.3 M each = 0.6
(E) 0.40 M C6H12O6 => 1 particle x 0.4 M = 0.4
ANSWER = (B) will have the highest boiling point