Choice B is the best answer. In the first sentence of the passage, Smith relays a claim: "We are told that it is not within the 'province of woman,' to discuss the subject of slavery; that it is a 'political question,' and we are 'stepping out of our sphere,' when we take part in its discussion." In the next sentence, Smith rejects this claim: "It is not true that it is merely a political question, it is likewise a question of justice, of humanity, of morality, of religion." She then argues that the subject of slavery "involves considerations of immense importance to the welfare and prosperity of our country, enters deeply into the home-concerns, the every-day feelings of millions of our fellow beings" and expands upon this point by providing an example of the difference, under slavery, between laborers who are enslaved and those who are within the "dignity of conscious manhood." Therefore, the best summary of the first paragraph is that Smith rejects a claim and elaborates on her reasons for doing so.
Choice A is incorrect. Although Smith may outline a conventional viewpoint in the first paragraph, she doesn't present evidence to support it. Choice C is incorrect. Although Smith introduces her subject in the first paragraph, she doesn't provide historical background for understanding it. Choice D is incorrect. Although Smith does identify a problem in the first paragraph, she doesn't propose steps to remedy it.