Final answer:
Reformers during the Progressive Era successfully lobbied for the Seventeenth Amendment, which mandated the direct election of U.S. senators to reduce government corruption and increase democratic participation.
Step-by-step explanation:
Reformers of the Progressive Era sought to reduce corruption in government by adopting a constitutional amendment that provided for the direct election of United States senators, which is represented by option (4). Prior to this reform, senators were selected by state legislatures, a process susceptible to corruption and political maneuvering. To rectify this and enhance democratic involvement, the Seventeenth Amendment was proposed and ratified, mandating that senators be elected directly by the people, thus giving citizens a more direct voice in their government. This was one of several reforms during the Progressive Era aimed at making the government more responsive to the will of the people and curtailing the power of political machines and party bosses.