The correct answer is C. Sample survey
Step-by-step explanation:
A sample survey is a study method that involves selecting a portion of a population and asking questions to these individuals to know their opinions or insights about a particular situation. Additionally, the answers provided by the selected individuals are used to make conclusions about all the population. This method is the one used in the situation described because the store manager selects only some customers to know about the sales they prefer and would likely use this information to know the preferences of all the customers.