Concept:The phrase
on edge means feeling anxious, tense, or nervous.
Explanation:In the given sentence, Susan was "on edge" after that incident.
This idiom shows that she felt mentally tense and emotionally unsettled because of what happened.
A person who is on edge is easily worried or startled.
It does not mean confused, surprised, or unhappy.
For example, waiting for important news can keep someone on edge all day.
So, the best word to replace "on edge" in this context is "nervous".
Answer:C. nervous