Answer: The participial phrase is "Waiting in the cafeteria line" and the modified word is the noun "girls".
Explanation: A participial phrase is a group of words that is introduced by a present participle or a past participle. While present participles end in -ing, past participles can end in -ed, -en, -d, -t or -n. Moreover, in order to express a full thought, participial phrases modify another word, which is generally a noun. In that way, participial phrases function as adjectives. In the case of this sentence, the participial phrase "Waiting in the cafeteria line" begins with a present participle ("waiting") and it is modifying the noun "girls" since the sentence can be rephrased into "The girls waiting in the cafeteria line talked about their history assignment".
