Which salt forms a solution with a pH < 7 when dissolved in water

A) NaCH3COO
B) RbI
C) LiNO2
D) (NH4)2SO4

You're looking for an acidic solution, so you need an acid present. The only one that will dissociate into an acid is D, which will give you NH4+, an acid that can lose a proton to solution. It's not a strong acid but the solution will be slightly acidic.