What quotation from Poe's "The Tell-Tale Heart" exemplifies the narrator's type of diction?

The term diction simply refers to the author's word choice. Authors can choose formal or informal words, ones with Latinate or Germanic roots, and common or rare terms. Another important choice in types of diction is the degree to which an author uses adverbs and adjectives. 

The narrator of "The Tell-Tale Heart" is insane. Poe conveys this by using very disjoint language, often consisting of short clauses lacking conjunctions (a rhetorical figure called asyndeton)....

The term diction simply refers to the author's word choice. Authors can choose formal or informal words, ones with Latinate or Germanic roots, and common or rare terms. Another important choice in types of diction is the degree to which an author uses adverbs and adjectives. 

The narrator of "The Tell-Tale Heart" is insane. Poe conveys this by using very disjoint language, often consisting of short clauses lacking conjunctions (a rhetorical figure called asyndeton). A good example of the narrator's characteristic diction is the opening of the story:

TRUE! --nervous --very, very dreadfully nervous I had been and am; but why will you say that I am mad?

The use of dashes conveys the narrator's inability to think in continuous rational sentences, and thus conveys his nervous character. He uses the words "nervous" and "mad" repeatedly in the story, word choices that focus the reader's attention on his mental state. The use of the adverb "dreadfully" and the use of the past perfect suggest a somewhat educated or intelligent speaker.