Tremulous means shaking or quivering; it also could mean timid
It makes the magical details sound more real.
The correct option for this question is:
A. “Take a big sip of this lemonade,” Jacobs grandmother cried out.
An oxymoron is a figure of speech in which apparently contradictory terms appear together. In this case "big sip" is the combination, since usually a sip is small, so a big sip sounds like something contradictory.