Final answer:
CaCl2 forms the least acidic solution when dissolved in water as it is derived from a strong base and a strong acid and does not increase the solution's acidity.
Step-by-step explanation:
The compound that would form the least acidic solution when dissolved in water among FeI3, NaHSO4, H2S, and CaCl2 is CaCl2. Calcium chloride is a salt derived from a strong base (Ca(OH)2) and a strong acid (HCl).
When it dissolves in water, it does not contribute any additional H+ ions to the solution, thus having a neutral effect on the pH. In contrast, the other compounds derive from weak bases or contain acidic hydrogen atoms that can increase the acidity of the solution.