Final answer:
In Jack London's "To Build a Fire," the man's biggest character flaw is that he lacks imagination.
Step-by-step explanation:
In Jack London's "To Build a Fire," the man's biggest character flaw, according to the narrator, is that he lacks imagination. This is evident throughout the story as the man fails to recognize the dangers and risks of traveling alone in such extreme conditions. He underestimates the severity of the cold, ignores advice from others, and fails to adequately prepare for emergencies. These actions demonstrate his lack of imagination and ultimately lead to his downfall.