какой ответ

The correct option to complete the sentence "Books are not to be taken ..." is "out".

Here's the explanation:

• The phrase "take out" is commonly used to refer to removing something from a designated place. In libraries, "taking books out" means borrowing them to read outside the library. Therefore, the sign indicates that books should not be removed from the library.

So the sentence reads: "The sign in the library reading room said: 'Books are not to be taken out.'"