Explanation
Correct answer: D.
Choice D is the best answer. The sentence presents the significance of the Hart-Celler Act to an audience unfamiliar with the history of US immigration, noting that the 1965 act abolished the national-origins quota system and explaining why that mattered, historically: because the old quota system had favored immigrants from northern Europe.
Why the other choices are wrong
Choice A
Choice A is incorrect. The sentence describes an aspect of immigration policy at the time the Hart-Celler Act was proposed; it doesn’t present the significance of the Hart-Celler Act to an audience unfamiliar with the history of US immigration.
Choice B
Choice B is incorrect. The sentence describes an aspect of immigration policy before the Hart-Celler Act; it doesn’t describe or present the significance of the act to an audience unfamiliar with the history of US immigration.
Choice C
Choice C is incorrect. While the sentence indicates that the Hart-Celler Act abolished the old quota system, it doesn’t explain the act or the quota system to an audience unfamiliar with the history of US immigration.