B. The Voting Rights Act of 1965
Explanation: (disclaimer: most of answer shown is part of a single, yet credible source)
The Voting Rights Act of 1965 was aimed at overcoming legal barriers at the state and local levels that prevented African-Americans from exercising their right to vote under the 15th Amendment. Shortly following the American Civil War, in some states of the former Confederacy, African-Americans rose to a significant level of power, becoming an "almost-majority" in the eligible voting population. Nevertheless, there was strong opposition to the extension of the franchise to African-Americans, and towards the end of the Reconstruction Period (1877), the Supreme Court, through the now-overwhelmingly-powerful White authorities, limited voting registration and turnout among African-Americans. As the vote in the White "minority" came to a head, the Black population saw a disproportionate change in the American political climate. Intimidation, fraud, and dread soared. Some examples of these restrictions include whites-only primaries, poll taxes, and literacy tests.
(Source: https://www.britannica.com/event/Voting-Rights-Act)
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I believe this choice most likely contributed to the rise in the African-American RVs between 1964 to 1969.
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**For context: as you may have heard in the news recently, the new, Republican-backed "restrictive voting laws" have come under fire as many hearken back to this idea of the oppressive voting culture of the Jim Crow South. **More reading: https://www.npr.org/2021/08/13/1026588142/map-see-which-states-have-restricted-voter-access-and-which-states-have-expanded | Democrat example: https://www.vanityfair.com/news/2021/07/democrats-are-freaking-out-over-the-gops-restrictive-voting-laws | Republican example: https://www.foxnews.com/politics/texas-senate-passes-controversial-voting-reform-bill ||
Other Choices:
A - Ratified in 1870, the 15th Amendment granted African-Americans and other minorities the right to vote and disbarred others from denying these people the chance to vote. As it is stated in Section 1: "The right of citizens in the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude."
Needless to say, however, this did not stop the abhorrent and disgraceful attitudes and practices put forth in the 1960s and even before that. Though this somewhat fits into what the question is asking for, it is likely not the best fit.
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C - Ratified in 1865, the 13th Amendment abolished slavery and involuntary (compulsory) servitude as a form of practice in the United States. As it reads: "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction." The underlined portion of Section 1 is still controversial today as it basically says "yeah, slavery is gone BUT..."
One particular viewpoint that explains a bit about this controversy is U.S. Senator Jeff Merkley (D-OR): "This country was founded on the beautiful principles of equality and justice---principles that have never been compatible with the horrific realities of slavery and white supremacy. The loophole in our constitution's ban on slavery not only allowed slavery to continue, but launched an era of discrimination and mass incarceration that continues to this day." (Source: https://www.kdrv.com/content/news/Sen-Merkley-renews-push-to-close-loophole-in-the-13th-Amendment-574665601.html) ((more information about his push to end that underlined portion, once and for all)).
This doesn't seem to be at least closer to the correct answer.
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D - The Civil Rights Act of 1964 is a landmark civil rights and labor law in the United States that outlaws discrimination based on race, color, religion, sex, national origin, and later, sexual orientation and gender identity. It prohibited the unequal application of voter registration requirements, racial segregation in schools and public accommodations, and employment discrimination. (Source: https://en.wikipedia.org/wiki/Civil_Rights_Act_of_1964) ((I know Wikipedia can have its doubts, but it seemed good))
Again, this choice feels like it could have been the right one. But it doesn't directly have an effect on the statistical question.
Sorry for the very LONG response, but there is so much more to this question than a simple multiple-choice answer.