Based on the sentences from ‘The Cask of Amontillado’, the
literary technique that is employed in the sentence is foreshadowing. This is a
way of having a writer or author give its reader an idea of what would likely
occur later on.
Think of an equation:
200 - 91 = 109
You can do anything to this equation, as long as you do the same thing to both sides. So, if you add 91 to both sides, you will get:
200 = 200
Since this holds true, Susan's strategy to add her answer to 91 makes sense.
The answer would be C.
(hope this helped)