Answer: Option (a) is the correct answer.
Step-by-step explanation:
A substance that does not get consumed in a chemical reaction and helps in increasing rate of the reaction by lowering the activation energy is known as a catalyst.
Hence, when the chemist finds that reaction speeds up when platinum is added into the reaction then the chemist must calculate or measure the mass of platinum before and after the completion of reaction.
This is because if there occurs change in the mass of platinum then it means that platinum is not acting as a catalyst in the reaction.
Thus, we can conclude that chemist must measure the mass of the platinum to determine whether it is a catalyst.