Final answer:
a) "The book that I borrowed from the library, which was a bestseller, was a great read." The sentence that contains both a defining and non-defining relative clause is option a) "The book that I borrowed from the library, which was a bestseller, was a great read."
Step-by-step explanation:
The sentence that contains both a defining and non-defining relative clause is option a) "The book that I borrowed from the library, which was a bestseller, was a great read."
In this sentence, the first relative clause 'that I borrowed from the library' is a defining relative clause because it restricts the meaning of 'the book' by specifying which book is being referred to. The second relative clause 'which was a bestseller' is a non-defining relative clause because it provides additional information about the book, but does not affect the essential meaning of the sentence.