Readers can determine the central idea of a text most effectively by identifying the text structure. Understanding how a text is organized can help readers pinpoint the main ideas and supporting details, which ultimately leads to a clearer understanding of the central idea.
While other options like restating the conclusion and defining key words can be useful, they may not always provide a complete picture of the central idea. Defining acronyms is generally a more specific task that may not contribute directly to grasping the overall central theme. Therefore, option 4. By identifying the text structure is the most relevant choice for determining the central idea.