Answer: Customers
Explanation:
According to the question,customers are the factor that are acting as external agent which are external part of Walmart store.External force of refusing to shop at Walmart stores that sell guns makes them change the flow of store even if they are not the part of store's internal factor.
Other options are incorrect because shareholders, employees and governing bodies are not related with the change occurring through external forces in store.Thus, the correct option is customers.