Which amendment was ratified to ensure that D.C. residents could vote for president?

The correct answer is the 23rd Amendment, which was ratified in 1961. This amendment granted residents of Washington, D.C., the right to vote in presidential elections. Before the 23rd Amendment, people living in the nation's capital were unable to participate in the electoral process for president because D.C. was not considered a state and therefore did not have electoral votes.

The 23rd Amendment allows D.C. to have a number of electors in the Electoral College equal to the least populous state, which ensures that D.C. residents have representation in selecting the president. This landmark change was significant in expanding the democratic rights of citizens living in the capital, addressing the principle of "no taxation without representation."

Other amendments mentioned do not pertain to voting rights in D.C. The 20th Amendment focuses on the terms of office for federal officials and the succession process. The 21st Amendment repealed Prohibition, and the 24th Amendment abolished poll taxes in federal elections. Each of these amendments serves a different purpose and does not relate directly to voting rights in Washington, D.C.