Irony is the literary element used in the Cast of Amontillado, specifically Dramatic Irony.
There are three types of Irony.
1) Verbal Irony - Using words. We say one thing but meaning another.
2) Situational Irony - Difference of what is expected to happen versus what actually happened.
3) Dramatic Irony - This is where the readers or viewers knew or is more aware of what is happening than the character in the story or play.
Extravagance or frugality
It is A. Decoration based on motifs such as flowers, fruit, or shell.