Answer:
C. an idea that the reader infers, because the author has not expressed it directly
Step-by-step explanation:
Unstated Idea is a Main idea in a passage or text, however, it is not directly or expressly stated in any given sentences in a passage. It is also known as Implied Idea. This means that instead of Main Idea being explicit stated, it is however only implied or inferred with the help of other vital information in the passage.
Hence, the correct answer is option C: an idea that the reader infers, because the author has not expressed it directly