Final answer:
A detail from 'To Build a Fire' that illustrates that survival on the Yukon trail was precarious is when the man's fire is extinguished by snow, leaving him vulnerable in the deadly cold.
Step-by-step explanation:
One detail from "To Build a Fire" that best supports the idea that survival was not guaranteed along the actual Yukon trail is the man's realisation of the severity of his situation after his fire is extinguished by falling snow. Despite his initial confidence, he faces the truth that "it was a matter of life and death with the chances against him" when he fails to build a fire successfully under extreme conditions. The narrative demonstrates the challenges of the harsh environment, notably when it describes how the man's fire is blotted out by snow from the tree under which he incautiously built it, emphasizing the unforgiving nature of the Yukon and the precarious balance between life and death.