Final answer:
NH₄⁺ is the ion that does not produce an acidic solution at 25°C.
Step-by-step explanation:
At 25°C, all of the following ions produce an acidic solution, except NH₄⁺. The other ions listed - HSO₃⁻, HPO₄²⁻, [Fe(H₂O)₆]³⁺, and [Al(H₂O)₆]³⁺ - all act as weak acids in water and, therefore, contribute to the acidity of the solution.