The answer is B: the carnival atmosphere encourages Fortunato to drink more wine.
In the short story "The Cask of Amontillado," by Edgar Allan Poe, Montresor takes advantage of the Carnival to make Fortunato want to go down to his wine cellar in the catacombs and get drunk.
Option A is incorrect because the reader knows from the beginning Montresor wants to take revenge. As regards alternatives C and D, Fortunato refuses to go back to the main floor - he wants to continue drinking.