To accurately determine which conclusion about Douglass's unutterable anguish is supported by paragraph 6, I would need the content of that specific paragraph. However, based on common themes in Frederick Douglass's work, I can suggest that a likely conclusion supported by Douglass's narrative is "He realizes that reading the book has shown him what he is missing by being enslaved." This aligns with his reflections on education and the deep pain of understanding the freedom and rights he was denied due to slavery.
To confirm this interpretation with certainty, please provide the text of paragraph 6 or any additional context.