Final answer:
NH₄Br forms an acidic solution when dissolved in water because ammonium ions can donate a proton to water, creating an acidic environment.
Step-by-step explanation:
The salt that forms an acidic solution when dissolved in water is NH4Br.
Ammonium bromide (NH4Br) is formed from the reaction of the strong acid HBr with the weak base NH3. When NH4+ dissolves in water, it can donate a proton to water, forming H3O+ and NH3. This reaction makes the solution acidic. In contrast, KCl, LiNO3, and Na3PO4 are salts derived from strong acids and strong bases, producing neutral solutions as neither their cations nor anions hydrolyze to affect the pH significantly.