Answer: The narrator has been insulted many times and now vows revenge against Fortunato.
Explanation: All other options are the result of a poor understanding of the text: in a void, the "thousand injuries of Fortunato" could be understood as injuries that Fortunato has suffered, but the context lets us know it is the other way around as soon as we read "I had borne," which actually means "I have suffered" (borne is the past participle of bear, not of born). To venture upon here means to start doing something new, figuratively coming into a new territory, which, in this context, is injury.
