The amendment that has had the greatest impact on increasing civil rights is D. Fourteenth Amendment.
The Fourteenth Amendment, ratified in 1868, grants citizenship rights and equal protection under the law to all individuals born or naturalized in the United States. It has been fundamental in various landmark Supreme Court cases related to civil rights, including those that addressed issues of racial discrimination, desegregation, and voting rights. Its Equal Protection Clause has been pivotal in advancing the civil rights movement and shaping the legal framework for protecting individual rights against state infringement.
In contrast, the other amendments listed—Twentieth-Seventh, Twelfth, and Twenty-First—do not primarily focus on civil rights or do not have a significant impact in that area.