A isnt the correct answer. B doesnt seem reasonable because its one specific item. D means they arent exactly a customer since they didnt buy anything therefore the answer would be C
This method is the best because it will survey the customers under one condition : every 10. If the store gets a lot of customers, then this should be easier to do. The rest are mostly dependent on what they do and that influences the survey. However, C makes it so that every 10 people are surveyed.
