The answer to this is letter A. In order to find the answer, you can just refer to the numbers given plus the given reaction. These will be your clues about what you should do to solve this. It can be confusing for a lot of people especially when the answers can be positive or negative.
It will depend on the given reaction and what is being solved. Is the initial reaction that is most important? Are you trying to look for the reaction rate or the time that is needed in order to get a reaction? These are details that you cannot overlook when you solve this question.