Answer:
C. The white boarder of her petticoat
Step-by-step explanation:
The narrator notices that ''her dress swung as she moved her body, and ......'' This is the evidence the writer gives about the narrator's attraction to his friend's sister. The narrator hardly speaks to her; he likes watching her walk to school. This is the reason why he describes her clothes . His likes the hem ( boarder) of her underwear ( petticoat) .