Choice D is the best answer. The word "tyrant" describes a cruel and unfair ruler. It is first used in the first sentence of the third paragraph, when Smith argues that in the event of a slave rebellion in the slave states, "the men of the North are bound to make common cause with the tyrant, and put down, at the point of the bayonet, every effort on the part of the slave, for the attainment of his freedom." The word occurs again in the seventh sentence of the last paragraph, when Smith asserts the strength of women's "aspirations that every inhabitant of our land may be protected . . . by just and equal laws" so that "the foot of the tyrant may no longer invade the domestic sanctuary." In both instances, the word "tyrant" is used to represent slaveholders and their allies. Thus, Smith's use of "tyrant" emphasizes the unjustness of slavery.
Choice A is incorrect because Smith's use of the word "tyrant" doesn't identify a specific individual as oppressive. Instead, it refers to all those individuals who profit from and abet the unjust institution of slavery. Choice B is incorrect because Smith's use of the word "tyrant" doesn't highlight the threat of aggression from abroad. Instead, it highlights national injustice. Choice C is incorrect because Smith's use of the word "tyrant" doesn't critique the limited roles for women in antislavery movements. Smith's use of the word "tyrant" refers to slaveholders and their allies, not those who would discourage women's participation in the antislavery movement.