The correct answer is the 14th Amendment. The details of the Slaughterhouse Cases are largely irrelevant. The most important thing to note is that the Slaughterhouse Court completely discarded the applicability of the “Privileges and Immunities” clause of the 14th Amendment, the text of which reads: “No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States.” In fact, the central holding of that case has never been overturned! In other words, the Supreme Court, to this day, STILL does not use the Privileges and Immunities Clause.

This should be a relatively easy question. The 15th Amendment granted the right to vote to all men. There is a good reason that the word “men” is emphasized in the question: many people learn, erroneously, that the 15th granted the right to vote only to black men, but this is incorrect (read the text of the amendment). Having said that, it realistically granted the right to black men (because white men largely already had the vote).

The District of Columbia is the location of our government. It is also a city in its own right. Until the passage of the twenty-third amendment while the citizens of the District of Columbia had the right to vote, they did not have any representation in the Electoral College. The passage of the amendment gave them status as a “state” in the Electoral College with three electors.

Although it may seem rather odd, the Constitution actually makes direct mention of civil rights only once – and this is not even done in the original body of the document but is instead contained as part of an amendment. The Fourteenth Amendment, which was passed after the Civil War, forms a special trifecta with the Thirteenth and Fifteenth Amendments; together, these Amendments offer explicit protection to every American, mainly through the preservation of the right to vote and the prohibiting of slavery. Neither of these two Amendments make direct mention of civil rights, however – that is left to the Fourteenth Amendment, which states that every American citizen, regardless of race, gender, sex, or religious belief, is entitled to all rights, fair treatment, and “equal protection of the laws.” In this manner, the Fourteenth Amendment is most famous for its so-called Equal Protection Clause, which played a crucial legal role in the Civil Rights Movement of the 1950s and 1960s.

The Bill of Rights, and the individual protections it ensures, has been almost universally extended to the State governments by a series of Supreme Court rulings. All of these rulings are drawn from the Court’s interpretation of the Fourteenth Amendment. The Amendment states, among other things, that state governments have no right to deny life, liberty, or property outside of the law of the land. This is called the due process clause.

The Seventeenth Amendment, passed in 1913, established that senators would no longer be chosen by a vote in the State Legislature, but would instead be elected by a vote of the people who lived in each senator's district. This represented the culmination of a decades-long struggle by the Progressives to extend greater suffrage rights and responsibilities to the people of the United States.

The passage of the 14th Amendment and the due process clause was interpreted by the Supreme Court to mean all persons have a right to equal due process, both before the federal and state governments. The federal government is supreme over state governments, and thus the states were forced to incorporate the Bill of Rights.

The Fourth Amendment is one of the most important parts of the Bill of Rights. It protects citizens from random government raids on their homes, and requires police to have probable cause or a court-sanctioned warrant to search a person's home. The Fourth Amendment is the reason the police, unless in an extenuating circumstance in which harm is imminent, need warrants to search homes and persons.

This answer requires relatively little explanation—it’s one of those questions that requires rote memorization. That said, there is a trick to remembering which amendment lifted prohibition (I believe we covered it in a previous question); 21 to drink—the 21st Amendment lifted prohibition.

As for the others, they are incorrect:

18th: Enacted Prohibition

19th: Granted the franchise to all women

20th: Does a number of things, primarily ending lame duck, and setting succession

23rd: Extends the franchise to residents of Washington DC (why was this necessary?)

The 15^th Amendment expanded the right to vote to people regardless of their race. The 19^th Amendment expanded the right to women. The 9^th Amendment reserves some unlisted powers in the Constitution for the states.