Choice A is the best answer. The fifth sentence of the last paragraph poses the following question: "Shall we silently behold the land which we love with all the heart-warm affection of children, rendered a hissing and a reproach throughout the world, by this system which is already tolling the death-bell of her decease among the nations?" In other words, the continuation of slavery in the United States is being criticized "throughout the world," such that the existence of slavery affects the United States by lowering the country's reputation in the international community.
Choice B is incorrect because Smith doesn't suggest that slavery affects the United States by leading many women to disavow their allegiance to the country. Instead, she suggests that it is partly women's patriotism that should stir them to protest slavery because it is lowering the reputation of the United States in the international community. Choice C is incorrect. Although Smith speaks ominously in the last paragraph of "the events of the last two years" that are "overclouding the bright prospects of the future," she doesn't cite any current violent conflicts in the country. Choice D is incorrect because Smith doesn't suggest that slavery weakens the authority of the country's government. Instead, she argues that it damages the country's reputation abroad.