This question assesses the ability to identify and correctly use relative clauses, which provide additional information about a noun without starting a new sentence. The correct answer is A, 'The book, which I borrowed from the library, is very interesting,' because it correctly uses a relative clause ('which I borrowed from the library') to add information about 'the book'. Options B, C, and D do not contain relative clauses; instead, they use coordinating conjunctions, participial phrases, and adverbial clauses, respectively, which serve different grammatical functions.