Final answer:
The girl is (D) hiking up a steep hill before she stops to rest, as indicated by the narrative describing the difficulty of the climb.
Step-by-step explanation:
The girl is hiking up a steep hill before she stops to rest. As described in the passage, the man lifts the girl and steps upward, closely attending to the nature of the path under his feet.
The text speaks of avoiding a misstep and keeping direction, highlighting the toil of climbing. The climb is depicted as strenuous due to the presence of words such as 'burdened,' 'frequent stops,' and 'bumped his knees.'
Thus, it is clear that the girl was involved in a physically demanding activity, which matches the option (D) Hiking up a steep hill.