Final answer:
The participial phrase in the given choices is '(Trying to understand,) Jem cannot reason how such an injustice could take place.'. Participial phrases act as adjectives and provide more detail to a noun. Other examples acted as nouns or were prepositional phrase.
Step-by-step explanation:
The phrase that would be considered a participial phrase is (Trying to understand,) Jem cannot reason how such an injustice could take place. A participial phrase is a type of verbal phrase that acts like an adjective in the sentence. In the example given, 'Trying to understand' modifies the noun ‘Jem’ and provides more information about him, making it a participial phrase.
Other examples in the choices such as '(Taking revenge)' and '(To be a hypocrite)' are gerund and infinitive phrases respectively. These phrases serve as nouns in the sentences. While '(of an African-American's ignorance)' is a prepositional phrase, that adds detail to a noun but doesn't work as an adjective.
Learn more about Participial Phrases