Final answer:
When barium nitrate is mixed with sodium sulfide, the precipitate formed is barium sulfide, BaS as barium sulfate is slightly soluble and nitrates are highly soluble.
Step-by-step explanation:
When barium nitrate, Ba(NO₃)₂, is mixed with sodium sulfide, Na₂S in solution, the precipitate that forms is barium sulfide, BaS. According to the rules of solubility and the reaction that takes place, barium sulfate (BaSO₄) is the only slightly soluble salt that could be formed in this scenario, and will precipitate out of the solution. This is due to the fact that nitrates like NaNO₃ are highly soluble and do not typically form precipitates. Therefore, the correct answer would be a.) BaS.