Final answer:
The author intensifies the rising action by introducing tension-building elements like the absence of an important character or vivid descriptions of danger, which lead to a high point of tension and conflict that is crucial before reaching the climax.
Step-by-step explanation:
The action by the British author that most significantly intensifies the rising action of the passage is the introduction of a new element or twist that increases the tension and builds up to the climax of the story. In the provided excerpts, this could be represented by a moment such as the absence of a key character when he is needed most (as in "Where is Mr. Bixby?" "Gone below, sir.") or by the vivid description of multiplying dangers. Such actions establish conflict and create a high level of suspense that pulls the reader further into the narrative and sets the stage for the climax, which is the story's turning point.
Rising action is a crucial part of a narrative structure that introduces secondary conflicts and builds tension. The vivid description of challenges or the sudden absence of a key character are examples of techniques used by authors to escalate this tension. The narrator's engagement with the audience in these moments is key to drawing readers into the story, just as Twain’s narrative does in the discussed passage.