Who had Stacy been visiting prior to her return last night?

The correct answer is that Stacy had been visiting her friend prior to her return last night. This indicates that she spent time with someone she shares a close relationship with, which is often a common activity when individuals take trips or visits. The context suggests that the visit was social in nature, highlighting the importance of friendships in personal life. This kind of interaction often brings enjoyment and can foster emotional well-being, making it reasonable for someone to visit a friend before returning home. The options represent different family and friend relationships, but visiting a friend specifically indicates the nature of her activities before her return.