Which literary element is responsible for the change of tone in the last couplet of the poem?

a)diction
b)irony
c)imagery
d)syntax

it was now deep dusk in the forest, and deepest in that part of it where these two were journeying.
as nearly as could be discerned, the second traveller was about fifty years old, apparently in the same rank of life as goodman brown, and bearing a considerable resemblance to him, though perhaps more in expression than features. still they might have been taken for father and son. and yet, though the elder person was as simply clad as the younger, and as simple in manner too, he had an indescribable air of one who knew the world, and who would not have felt abashed at the governor's dinner table or in king william's court, were it possible that his affairs should call him thither. but the only thing about him that could be fixed upon as remarkable was his staff, which bore the likeness of a great black snake, so curiously wrought that it might almost be seen to twist and wriggle itself like a living serpent. this, of course, must have been an ocular deception, assisted by the uncertain light.

"come, goodman brown," cried his fellow-traveller, "this is a dull pace for the beginning of a journey.
take my staff, if you are so soon weary."

"friend," said the other, exchanging his slow pace for a full stop, "having kept covenant by meeting thee here, it is my purpose now to return whence i came.
i have scruples touching the matter thou wot'st of."

"sayest thou so? " replied he of the serpent, smiling apart. "let us walk on, nevertheless, reasoning as we go; and if i convince thee not thou shalt turn back.
we are but a little way in the forest yet."

"too far! too far! " exclaimed the goodman, unconsciously resuming his walk. "my father never went into the woods on such an errand, nor his father before him.
we have been a race of honest men and good christians since the days of the martyrs; and shall i be the first of the name of brown that ever took this path and kept"