The collection of rare artifacts, along with several newly discovered manuscripts, are being prepared for exhibition at the museum, where it is hoped they will attract a significant number of visitors.
Correct: D
The subject of the sentence is 'The collection,' which is singular. Phrases like 'along with several newly discovered manuscripts' are parenthetical and do not change the number of the subject. Therefore, the verb must be singular: 'is being prepared,' not 'are being prepared.' This eliminates options A and C. In the latter part of the sentence, 'where it is hoped they will attract a significant number of visitors,' the pronoun 'they' correctly refers to the multiple items within the collection and the manuscripts (the artifacts and manuscripts collectively), which are the things that will attract visitors. Option B incorrectly uses 'it' to refer to both the collection and manuscripts, which would be awkward as 'it' typically refers to a singular entity. Option E changes the verb tense to 'attracts' and uses 'it' again. Option D correctly uses the singular verb 'is being prepared' for 'The collection' and the plural pronoun 'they' for the artifacts and manuscripts that will attract visitors.