The best response for what a main idea is would be:
a lesson that is not stated in a text
However, if you're looking for a clearer and more commonly accepted definition, the "main idea" typically refers to the primary point or message that the text conveys, which can sometimes be inferred rather than directly stated.
For the options you provided, none accurately encapsulate "main idea" without nuance, but the closest option would be:
a lesson that is not stated in a text.
Feel free to ask if you'd like more clarification or help!