C is the only right answer. There's no such thing as "childrens." "Children" is already plural, so if you want to make something belong to the children, you add apostrophe-s. So the section where the books for kids live is the "children's section." The same goes for "people's money"--the money belonging to the people. "People" is usually plural, but it can be singular when referring to a group of individual people who are one collective, such as the French people or the Jewish people. But then where there is more than one people, it can become plural like in D.