Final answer:
Jon Krakauer ends Chapter 16 of 'Into Thin Air' pondering his own reasons for deciding to climb Mount Everest in the first place.
Step-by-step explanation:
At the end of Chapter 16 in Into Thin Air by Jon Krakauer, the question that Jon Krakauer is left pondering is why he decided to climb Everest in the first place (option a). This introspective question comes after Krakauer has detailed the harsh conditions on Mount Everest, including extreme cold, low oxygen levels, and low barometric pressure, which make it an environment hostile to human life. Krakauer's experiences on the Everest expedition force him to reflect on his personal motivations and the events that unfolded during that fateful climb.