The effect of the repetition is to establish the poem's tone (option B).
In this poem, the narrator describes his terror feeling as some visitor is trying to enter the place. This idea is explained in some visitor entreating entrance at my chamber door which is repeated twice.
This repetition helps the author establish the tone or the general atmosphere in the poem. In this case, the tone can be described as anxious, nervous, or scared because the narrator is expecting someone to enter, and he does not know who this person is.
Based on this, the repetition establishes the tone.
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