Unclear reference occurs when a pronoun (often "it," "this," "that," or "they") could refer to more than one antecendent in a sentence. Hence, the answer is option B. This sentence does not contain an unclear reference.
Though, the pronoun present (they) is a plural pronoun which is taken as singular reference is still grammatical as this is acceptable in the spoken English. It is used instead of the referent 'this school district,' however, it is grammatical.
Therefore, the correct answer is option B. This sentence does not contain an unclear reference.
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