Final answer:
The story will most likely involve C. Auguste Dupin solving a mystery, as he is a prototypical detective character known for analytical prowess, and the text hints at his engagement with a case involving a stolen document.
Step-by-step explanation:
Based on the provided text, it is most likely that b) Dupin will solve a mystery. C. Auguste Dupin is a character created by Edgar Allan Poe, known for his sharp analytical abilities and for being a prototype for future fictional detectives. The text suggests that Dupin is about to engage in a discussion regarding a sensitive matter that involves a document of great importance which has been stolen, and it is known to still be in the possession of the thief. The reference to Dupin's analytical skills and involvement in such a case suggests that the story will likely involve him solving this mystery or a related one. His friend, the narrator, seems ready to assist or observe Dupin's methods, setting the stage for a detective story.