The third paragraph in this passage is the author stating that Taruskin relied on examining only one of the two ways that art is produced by and for the elite. Namely, commissioning an artist to produce a work of art that expresses the sensibilities of the elite. It ignores commissioning art that is made for the elite that the elite perhaps themselves may not have liked. It also ignores that artists could have subverted the ideals of the elite in their commissioned pieces. The author states that this is because Taruskin focusing on commissions that express the sensibilities of the elite allows analysis of the way that art embodied ideals of the elite.
This is why answer choice B is correct. In stating this explicitly, it is identifying the assumptions that Taruskin's analysis (mentioned in the first paragraph) relies on.
Answer choice D is incorrect because there is no conclusion reached in the second paragraph. In this light, it is impossible for anything to support a conclusion in the second paragraph.
I hope this helps. Please let me know if you have any other questions.