Most wolves range over a wide area in search of prey; this particular wolf hung around the same area. An explanation that immediately suggests itself is that this particular wolf found enough prey in this area, so it didn't have to run all over looking for food. This is the tack taken by (B): If the wolf had a large stable population of sheep on which to prey in the immediate vicinity, there was no need for it to range over a wide territory looking for food.
(A) doesn't have much direct bearing on this particular wolf's lack of mobility. While it's true that a wolf might find it harder to move around in mountainous country, the stimulus says that wolves in general tend to cover great distances in search of food. There's no hint that a wolf in a mountainous area should prove an exception to this rule.
(C) is irrelevant: While the White River Wilderness Area may once have supported a population of wolves, knowing this does nothing to explain the behavior of this particular wolf.
(D), if anything, gives what seems to be a reason for our wolf to make tracks and migrate somewhere else. Certainly (D) doesn't explain why our wolf didn't follow usual wolf hunting methods.
(E) answers the wrong question; it would help explain why the naturalists couldn't use our wolf to study the movements of the larger pack. However, we haven't been asked that; we want to know why this specific wolf didn't behave the way wolves usually do.
