Correct choice is (a) 1.0 M KOH
Easiest explanation: KOH→K^+ + OH^–
(NH4)2SO4→2NH4^+ + SO4^2-
K2CO3→2K^+ + CO3^2-
K2SO4→2K^+ + SO4^2-
Concentration of particles in 1.0 M KOH solution is minimum (= 2 M). Hence, it will minimum elevation in boiling point.