The correct answer is Option C. It is the same as the theme of the passage.
The central idea of a text is the author's aim to convey a message within its lines, and it always consists of the theme being discussed. Although often the author might not indicate the central idea explicitly, it can be carried out with clarity as the text goes on.
A theme is a very specific element taken from a subject, whereas a topic is a generalization of this subject. For example:
Topic: Books.
Theme: The Role of Books in a Student's Learning Process.
With this information, it is possible to tell the author means to describe how important books are for a student to learn new things as a central idea for their text.