Step 1: Understanding the possessive form.
The correct expression is "Children's books" because the word "children" is plural, and the possessive form is created by adding an apostrophe and "s" (children's) to show ownership of the books.
Step 2: Why other options are incorrect.
- (A) "Children books" is incorrect because the possessive form is missing.
- (B) "Childrens books" is also incorrect because "children" does not require an additional "s" at the end when it is in the possessive form.
- (D) "Child books" is incorrect because the word "child" is singular, and it does not correctly convey the idea of books belonging to children.
Step 3: Conclusion.
Thus, the correct expression is "Children's books".
Final Answer: Children's books.