Answer:
In this case, they are using a simple random sampling method, because in each store they choose 100 random people without stratifying them or put any condition. In the simple random sampling method, the samples have the same probability to be chosen, but it isn't reliable in big populations. The population is all the costumers that visit all the stores, while the sample is just the costumers that were surveyed in all the stores. So, it's a simple random sampling method, and the population is all the people that visit all the stores.