Final answer:
The 8,000 meter mark in climbing is important primarily because it marks the start of the 'death zone', where climbers often need supplemental oxygen to survive due to low air oxygen levels.
Step-by-step explanation:
The importance of the 8,000 meter mark in climbing is significant because it is commonly recognized as the beginning of the "death zone" in mountaineering. This answer is found considering the options provided: a) It marks the point where supplemental oxygen is typically needed due to the thin air and low oxygen level.
b) Climbers might have to turn back if they haven't reached certain altitude points by a specific time for safety reasons, though the 8,000 meter mark itself isn't a universal turnaround point. c) It does not indicate the halfway point to the summit of Everest. d) Contrary to becoming less challenging, the terrain above 8,000 meters is often extremely difficult and dangerous.