Final answer:
The 17th Amendment to the US Constitution established the direct election of US Senators by popular vote.
Step-by-step explanation:
The correct answer is 17th Amendment. The 17th Amendment to the United States Constitution, ratified in 1913, established the direct election of US Senators by popular vote. Before the 17th Amendment, Senators were chosen by state legislatures. This amendment shifted the power of choosing Senators from state governments to the people. It was a significant change in the democratic process of electing US Senators.
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