Final answer:
Sylvia's internal conflict is best exemplified by her dilemma over disclosing the heron's nest location, reflecting her struggle between self-identity and pleasing others.
Step-by-step explanation:
The detail that best develops Sylvia's internal conflict is her struggle to decide whether to reveal the heron's nest to the young man. This choice represents Sylvia's broader challenge of defining her identity and remaining true to herself versus pleasing others. Her internal monologue provides insight into this conflict, while the narrator's intrusion encourages the reader to sympathize with Sylvia's dilemma. By closely examining Sylvia's thoughts and feelings, readers can understand her internal struggle and the growth it spurs within her.