Final answer:
NH4NO3 forms an acidic solution because NH4+ is the conjugate acid of a weak base and partially hydrolyzes in water, producing H3O+ ions and lowering the pH.
Step-by-step explanation:
The student's question asks which of the listed compounds produces an acidic solution when dissolved in water. Among the given options, NH4NO3 (Option A) is the compound that will form an acidic solution. This is because the NH4+ cation is the conjugate acid of the weak base NH3, and it will partially hydrolyze in water to give an acidic solution. More specifically, NH4+ ions will react with water to produce H3O+ ions (hydroxonium ions) and NH3, thus lowering the pH of the solution.
The other compounds listed, such as K2S, Sr(NO3)2, Na2O, and NaF, either produce neutral solutions since their ions are derived from strong acids and strong bases (i.e., Na+ and NO3−), or in the case of NaF, produce a slightly basic solution because the F− ion is the conjugate base of a weak acid, HF.