Final answer:
Before the Seventeenth Amendment, U.S. senators were chosen by state legislatures, with the amendment changing this to a direct popular vote by the people.
Step-by-step explanation:
Before the Seventeenth Amendment was passed in 1913, United States senators were not chosen by a direct popular vote of the people in each state. Instead, they were selected by the state legislatures. This means that the correct answer to the student's question is D. The state legislatures.
The system of having state legislatures elect U.S. Senators was changed with the ratification of the Seventeenth Amendment, which established the popular election of senators by the people of the states, responding to issues with scandalous elections and the desire for more democracy.