Correct Option (A)
The passage explicitly states, "A thin, shy girl, her knees quaking, her stomach churning in terror." This directly indicates that Cynthia's knees were quaking as a physiological manifestation of her extreme nervousness and terror, consistent with her described shy nature and apprehension about performing on stage.
Incorrect Options:
The provided text does not mention any instance of the teacher scolding Cynthia. Her distress is solely attributed to the prospect of performing.
While Cynthia is described as "thin," the passage presents this as a descriptive characteristic rather than the cause of her knees quaking. The narrative attributes the quaking to "terror."
Cynthia's fear is primarily directed towards the act of standing on stage and delivering dialogues, as indicated by "the mere thought of it made her feel sick." While performing "in front of the entire class" contributes to her stage fright, the fundamental fear is of the performance itself, not specifically of her classmates as individuals.