In "The Cask of Amontillado," the line that conveys a tone of sarcasm is:
"You? Impossible! A mason?"
This line is sarcastic because Montresor is pretending to be surprised or disbelieving that Fortunato could be a mason when, in reality, he is using this statement to mock or belittle Fortunato. The use of the word "impossible" adds to the sarcastic tone because it implies that it's inconceivable for Fortunato to be a mason, which is meant to be taken in a mocking or derisive manner.
Sarcasm is a form of verbal irony where someone says something but means the opposite for humorous or critical effect. In this case, Montresor's statement is meant to be taken as sarcastic because he knows Fortunato is a mason, but he is pretending otherwise to taunt him.
