Answer:
The detail that best introduces the theme of sacrifice is "Here was five-foot-five of exquisite, blooming, twenty-year-old American girlhood sending away the man she confessed to care for, because, forsooth, she would not marry before her elder sister!" (in option 5).
Step-by-step explanation:
This detail emphasizes the sacrifice made by the young girl in rejecting the man she cares for in order to prioritize her elder sister's marriage. The reference to "sending away" the man and the phrase "forsooth," which means "in order to comply with a request or demand," indicates that the girl is making a selfless sacrifice. This detail introduces the theme of sacrifice by highlighting the conflict between personal desires and familial obligations.