Answer: I would contend that the right answer is actually "at fifty below."
Step-by-step explanation: Just to elaborate a little on the answer, it can be added that on this excerpt from Jack London's short story the character is on his way to Henderson Creek, but the narrator is not describing this area yet. The only direct and precise reference to the physical environmental conditions of the story at this point is the one about the temperature. This suggests that the character is in an extremely cold landscape.