Answer: 2/3 books on politics
Explanation:
The opportunity cost of something is the next best alternative that you would have taken if you did not instead go for the option that you took.
Opportunity cost here is the number of books on politics that Bill had to give up to get a single book on history.
= Number of politics books / Number of history books
= 4 / 6
= 2/3 books on politics
Bill gave up 2/3 books on politics to get a single history book.