Final answer:
The author's statement that Balto was not chosen for the relay team helps predict Balto's later heroic role in successfully transporting the serum.
Step-by-step explanation:
The clue in the text that best helps a reader predict that Balto will play a heroic role in successfully transporting the serum to Nome is when the author points out that Seppala did not choose Balto for his relay team. This suggests that Balto has qualities that make him an unexpected hero, as he was not initially selected for the task. This builds anticipation and foreshadows the courageous role that Balto will eventually play in the story.
Learn more about Foreshadowing in literature