The option that best supports the idea that Tenzing Norgay might not be a reliable narrator is: "In regard to this, I must say in all honesty that I do not think Hillary is quite fair in the story he later told, indicating that I had more trouble than he with breathing and that without his help I might have collapsed. In my opinion our difficulties were about the same—and luckily never too great—and we each helped and were helped by the other in equal measure."
This passage shows that Tenzing's version of the story is not the only one, that Hillary tells it differently. So the reader can't believe Tenzing 100%.