Answer:
The main character stopped trying to build a fire or get back to the camp.
Step-by-step explanation:
To Build a Fire is a short story written by Jack London. There are two versions of this story, and the latter one, written in 1908, is the most known.
This version tells about an unnamed male protagonist who, followed by a dog, decides to go on the journey along the Yukon trail. He ignores the warnings of an elderly man about the dangers of hiking alone in extreme cold and at one point begins to freeze to death. After building a fire and leaving it to continue his journey, he later attempts to build another but fails to do so. Desperate, he then tries to kill the dog to gain access to its body heat but doesn't succeed, after which he dies of hypothermia.
This is the story of a man's unsuccessful journey. The detail that best supports Jackson's topic sentence is the first one: The main character stopped trying to build a fire or get back to camp. The fact that the protagonist failed to build a fire or get back to the camp is the direct cause of his death.