Take a look at the sentence again:
The hike through the canyon exhausted Kim. The steep incline made her thigh
muscles throb, __________, and the relentless sun burned her skin.
The blank requires a short main clause. You chose shoulders that ached from the heavy pack, which is a noun followed by an adjective clause. You thus have a parallelism error. Before you continue, review the rules for parallel structure.
Go back to the sentence to try again.