Final answer:
The advertising agency should C. choose a simple random sample from people who have bought the new MP3 player to determine its popularity.
Step-by-step explanation:
The method the advertising agency should use to collect information on the popularity of the new MP3 player is option C: Choose a simple random sample from people who have bought the product. A simple random sample involves selecting a group of individuals from the population in such a way that every individual has an equal chance of being chosen.
In this case, the agency should randomly select a subset of people who have already purchased the MP3 player to gather information on its popularity. This method would provide a representative sample of the population of MP3 player owners and allow the agency to gather insights into the product's popularity based on the feedback of actual customers.