Answer:
I would select "that they" as the error for this sentence.
Explanation:
When the author of the sentence chose to use the pronoun "they", he/she made the sentence ambiguous. There are two possible antecedents for the pronoun:
Members of the community donated books to the new charter school in the hope that they would be a success.
Since the third-person plural pronoun "they" can refer to both "members" and "books", which are third-person plural nouns, the sentence is unclear. Who or what would be a success? The members would be a success? The books would be a success? Let's see two possible changes to correct the sentence:
1. Members of the community who hoped to be a success donated books to the new charter school.
2. Members of the community donated books to the new charter school in the hope that the books would be a success.
