The correct option is B the
The article 'the' is used before a specific, definite noun. The sentence given clearly states that a specific book is spoken about. Since a book has only one back, option B is correct. A and an being indefinite articles are incorrect answers here since they're usually used for nouns that are not specific or in a general context, hence options A and C are incorrect. The 'back' of 'the book' being a specific countable noun, an article is required ahead of it, which means the option D is incorrect.