The correct answer is "a".
The indefinite article "a" is used before singular nouns that refer to a general or unspecified thing. In this case, the speaker is referring to a book in general, not a specific book.
The indefinite article "an" is used before singular nouns that begin with a vowel sound. In this case, the noun "book" does not begin with a vowel sound, so "an" is not the correct answer.
The definite article "the" is used before singular nouns that refer to a specific thing. In this case, the speaker is not referring to a specific book, so "the" is not the correct answer.
The answer "none of these" is not correct because the speaker is using the indefinite article "a".
