The sentence that contains parallel structure is the following one: The cat stops, streches, and yawns before attacking the dog.
Explanation:
Parallel structure, also called parallelism, is the repetition of a chosen grammatical structure within a sentence. By making each compared item or idea follow the same grammatical structure, you create a parallel construction. In the sentence above, each verb phrase in the main clause is in the present simple tense (stops, streches, yawns). As regards, "attacking", it is a gerund which functions as the object of the PP "before".
