Answer:
The last method provided in the question is correct.
Explanation:
By determining the time consumed for the liquid to become clear when X and Y are combined will permit the researcher to know the time of reaction and the progression of the rate of reaction. Post that similar method should be performed when the substance A is supplemented to the mixed liquids X and Y. If the reaction takes place at a brisk rate, then it can be confirmed that A functions as a catalyst for the reaction.