Final answer:
The Progressive Era followed the decline of the Populists and focused on social and political reforms to address the challenges of industrialization and urbanization.
Step-by-step explanation:
After the decline of the Populists, the key reform movement that emerged was the Progressive Era, which began in the early 20th century. This era was marked by significant social and political reforms aimed at addressing the issues of industrialization and urbanization that had risen to prominence in American society. The Progressive movement drew on many Populist ideas, such as the importance of governmental intervention in the economy, but focused more on issues such as consumer protection, labor rights, and urban reform. The Progressives successfully pushed for reforms that changed regulatory environments and increased the efficiency of government, thus shaping American politics and policies for years to come.