Answer:
a. to define the topic
Explanation:
The central idea of a text is the main idea that the author focuses on throughout the text and defines the topic.
For example, the title or topic of a book could be "the effects of video games on kids, effects, causes, and possible solutions". As a result of this, the reader invariably knows that the central idea of the book is video games, and this goes on to show how the central idea in a text helps to define the topic.
