The correct answer is C. girls.
A participial phrase is composed by a participle and other words such as nouns, pronouns, noun phrases or prepositional phrases. When the participial phrase is placed at the beginning of the sentence, it modifies the noun that immediately follows it.
In the sentence "[s]hrieking from in the tenth row, the girls were no doubt his biggest fans.", "shrieking from in the tenth row" is the participial phrase because it starts with a participle. As it is at the beginning of the sentence, the noun modified by the participial phrase is the one that immediately follows it, which is "girls".