The effects of dramatic irony in the passage are most clearly shown in option D: "readers understand why Rebecca tries to divert Captain Barrington by saying she is mending the overcoat for a townsman in need." This option highlights the contrast between what the audience knows about Rebecca's true intentions (helping the Continental Army) and what Captain Barrington believes (that she is simply mending clothing). This creates a layer of tension and engagement for the readers, who are aware of the underlying truth that the character does not see, showcasing the essence of dramatic irony.
While the other options reflect various aspects of character actions and plot developments, option D specifically captures the audience's awareness of Rebecca's motives and her strategic deception, which is a key element of dramatic irony.