The correct answer is C. A flock of crows perched on a wire while they cackled and called their night songs.
Explanation:
In grammar, the key elements in a prepositional phrase are a preposition such as until, after, by, on, etc. and a noun or object used to complement this preposition. This occurs if the section "on a wire" is added because this includes the preposition "on" and its object "wire". Additionally, prepositional phrases often describe a noun; in this case, they provide more information about the flock of crows. Thus the sentence that adds a prepositional phrase is sentence C.