Which amendment changed the selection process for U.S. senators?

The amendment that changed the selection process for U.S. senators is the 17th Amendment. Ratified in 1913, this amendment established the direct election of senators by the voters of the states, replacing the previous method where senators were selected by state legislatures. This shift was rooted in the Progressive Era's push for greater democracy and accountability in government. By allowing the public to vote directly for their senators, the amendment aimed to reduce corruption and ensure that senators were more responsive to the needs and opinions of their constituents.

The other amendments listed do not relate to the selection process of senators; instead, they address different issues such as voting rights, taxation, and prohibition. The 15th Amendment deals with voting rights regardless of race, the 16th Amendment authorizes income taxation, and the 18th Amendment instituted Prohibition. Thus, the 17th Amendment is the correct response, as it directly impacted how senators are elected and is significant in the context of increasing democratic participation in the U.S. political system.