Final answer:
The sentence builds tension by revealing a plot of mutiny and the challenge of not knowing the identities of the conspirators, thus foreshadowing conflict and creating suspense.
Step-by-step explanation:
The underlined sentence in paragraph 8 from the passage builds tension because it introduces the element of a secret plot that characters must respond to, and there is uncertainty regarding the identity of the conspirators. The phrase "I have overheard, of late, some of the rascals plotting to murder the officers and take this ship" reveals a plot of mutiny and murder, increasing the stakes of the narrative and foreshadowing potential conflict. The tension is further heightened by the narrator's admission of being unable to recognize the culprits, which creates a sense of suspicion and the imminent need for action to prevent the disastrous outcome.