Final answer:
The question pertains to a literature class and involves Gatsby, a character who experiences nervousness. The specifics of what Gatsby knocks off the mantle are not provided in the excerpts, but the themes of tension and the physical or environmental setting are prevalent.
Step-by-step explanation:
The question appears to relate to a literature class, where the student is asking about an event in a narrative involving a character named Gatsby. Specifically, the question is referencing a moment of nervousness experienced by Gatsby, wherein he leans upon a mantle and consequently knocks something off.
However, based on the information provided, the object that Gatsby knocks off is not explicitly mentioned in the excerpts given. The references within these excerpts do not directly answer the question about Gatsby but suggest a range of scenarios depicting tension, physical states, or environmental settings from different narratives, which could be from various works of fiction or literature.
Understanding these contexts could potentially help the student in exploring themes such as apprehension, anticipation, or the consequences of actions within literature.