All the customers who entered a store on a particular day were given a survey to fill out concerning their opinions of the service at the store. What type of sample is this?

In the given case, we can clearly see that no efforts are made in order to reach customers all over the region in which the store is located.

This makes it a form of convenience sampling.

The research might have biased results because the customers who are surveyed are already visiting the store and hence it is likely that they found a satisfactory/good service at the store otherwise they wouldn't have come.

The survey should also have reached the customers who used to visit the store initially but no longer does now.

Hence, the survey is being conducted for some specific people according to the convenience of the surveyor which makes it a form of convenience sampling.