Caribou are wary animals with excellent hearing, so stalking them over the treeless landscape, getting close enough to kill it with nothing but a handheld lance, as Dorset people did, required exceptional hunting skill.

1. so stalking them over the treeless landscape, getting close enough to kill it
2. so to stalk them over the treeless landscape and get close enough to kill one
3. so in order to stalk them over the treeless landscape and get close enough to kill one
4. and so in order to stalk it over the treeless landscape, getting close enough to kill it
5. and so stalking them over the treeless landscape and getting close enough in order to kill it

刚查了字典so可以作为连词，字典上给出的例子:So that's how I first met your mother.

然后我觉得one在这里有强调的意思，题目的意思是c是一种很警觉的动物并且听力很好，想要抓到“一只”都不是一件容易的事情，需要出色的hunting skill

答案是B吗？

B is the best answer!

SO serves as a conjunction in this case, as is very common!

IT here doesn't have a tangible and specific referent, while ONE best fits in.