The correct definition of a "central idea" among the options provided is:
1. A concept introduced in the middle of a text.
However, it's important to clarify that the central idea often encompasses the main themes or messages of the text, and it can be more broadly understood as the primary point or focus that the author wants to convey throughout the work, which may be introduced earlier or developed gradually.
If you'd like to understand more about central ideas and themes or need additional information, feel free to ask!