Answer:
The correct answer is the option C: what people have done.
Explanation:
To begin with, in order to accomplish that type of analyzes the researcher will need to carefully investigate about what the people have done in the past regarding the subject of interest such as, purchase behavior and post purchase behavior as well. Therefore that, in that order, the most important information to gather is the one that clarifies what the people have done in the past so in that way the researcher will be able to understand them better.