Final answer:
The sentence 'The book____ you gave me is very interesting', can be correctly completed with either 'that' or 'which'. Both are correct relative pronouns that make the sentence gramatically complete.
Step-by-step explanation:
In the sentence 'The book____ you gave me is very interesting', you could use either 'that' or 'which' as the relative pronoun. Both are correct and convey the same meaning. For example, 'The book that you gave me is very interesting.' or 'The book which you gave me is very interesting.' These relative pronouns are often interchangeable in relative clauses, depending on your preference in style and context.
Learn more about Relative Pronouns