Final answer:
option c.The sampling method where a store clerk asks a customer to complete and mail back a questionnaire is known as convenience sampling.
Step-by-step explanation:
When a store clerk asks his customer to fill out a questionnaire and mail it back, the type of sampling method being used is convenience sampling. This method involves using results that are readily available, typically from individuals who are easily accessible or willing to participate. It's important to note that convenience sampling is a non-random method and can often produce biased data as it does not give each member of the population an equal chance of being included in the sample.
The store clerk asking customers to fill out a questionnaire and mail it back is an example of convenience sampling. Convenience sampling involves using readily available results, such as interviewing potential customers in a store. In this case, the store clerk is selecting customers who are already in the store browsing through available products.