Answer:
Since we are to choose only what in inferred in the story, the correct option is number 2. One reason Montresor is wearing a costume is to be sure no one recognizes him when he is walking back to his house with Fortunato.
Step-by-step explanation:
"The Cask of Amontillado" is a short story by Edgar Allan Poe. The narrator is Montresor, a man who wants revenge against Fortunato. It is clearly stated at the beginning of the story that Fortunato has offended Montresor:
THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Therefore, number 6 is eliminated. Since it is carnival, Fortunato is wearing a costume:
He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells.
We can eliminate number 1 with that information. However, it is not stated anywhere that Montresor was wearing a costume with the purpose of disguising himself. So, we should check number 2 as an inference.
Montresor leads Fortunato to the catacombs and gives him wine to keep him drunk:
We came at length to the foot of the descent, and stood together upon the damp ground of the catacombs of the Montresors. [...]
Here I knocked off the neck of a bottle which I drew from a long row of its fellows that lay upon the mould.
"Drink," I said, presenting him the wine.
We can, thus, eliminate numbers 4 and 8. We can also eliminate number 5, since the narrator clearly states the time:
It was now midnight, and my task was drawing to a close.
Finally, we can eliminate number 3 as well. Fortunato went to the catacombs with Montresor because he was a proud man when it came to knowing wines. He was jealous of the possibility that Luchresi would be the one tasting the Amontillado:
He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine.
"Let us go, nevertheless. The cold is merely nothing. Amontillado! You have been imposed upon. And as for Luchresi, he cannot distinguish Sherry from Amontillado."