The correct answer is: D. no article
The word “book” is a common noun, which means it refers to a general class of things. In this case, the speaker is not referring to a specific book, but rather to any book that they might find in the library. Therefore, they do not need to use an article.
The article “a” is used before singular common nouns that refer to a specific thing. For example, if the speaker said “I want a book about dogs,” they would be referring to a specific book about dogs that they have in mind.
The article “an” is used before singular common nouns that begin with a vowel sound. For example, if the speaker said “I want an apple,” they would be referring to a specific apple that they have in mind.
The article “the” is used before singular common nouns that refer to a specific thing that has already been mentioned or that is unique. For example, if the speaker said “I want the book that I borrowed from you last week,” they would be referring to a specific book that they have already borrowed from the listener.
In this case, the speaker is not referring to a specific book, so they do not need to use an article.