C. The horses pulled the wagon up the hill, trotted down the other side, and then paused to rest at a stream.
Explanation:
A sentence with a parallel structure concerns the repetition of a chosen grammatical form to show that the ideas presented are equally important and to give a natural flow to the reading. This construction uses words, phrases or clauses that are grammatically similar in structure, sound, meaning, or meter, like the sentence from option C, which contains clauses that repeat the same verbal tense: the simple past tense (“pulled,” “trotted” and “paused”).
