Final answer:
K2S will conduct electricity in water, while CH4, FeCl2, and SO2 will not.
Step-by-step explanation:
Out of the given substances, the only one that will conduct electricity in water is K2S (potassium sulfide). This is because K2S dissociates into ions when it dissolves in water, specifically potassium ions (K+) and sulfide ions (S2-), which are capable of carrying an electric charge. The other substances, CH4 (methane), FeCl2 (iron(II) chloride), and SO2 (sulfur dioxide), do not dissociate into ions and therefore cannot conduct electricity in water.