Final answer:
Metal X, being lower than metal Y in the activity series, is less active and thus, can't replace metal Y in a solution.
Step-by-step explanation:
If metal X is lower than metal Y in the activity series, then metal X is less active than metal Y. When comparing metals in the activity series, the metal that is higher in the series will replace the metal that is lower in a single-replacement reaction. Therefore, the correct answer is c. is less active than metal Y. Metals lower in the activity series do not replace ions of metals that are higher in the series, and they do not form positive ions as readily as metals that are higher in the series.