In the following question find out the alternative which will replace the question mark?
Butter : Milk: : Book: ?

Paper

The answer is Paper.

The analogy is between butter and milk, and a book and paper. Butter is made from milk, and a book is made from paper. So, the missing word must be the material that a book is made from. Of the answer choices, only paper fits this description.

The other answer choices are not relevant to the analogy. Author is the person who writes a book, printing is the process of making books, and grains are the raw materials used to make paper. None of these are the material that a book is made from.