Final answer:
The Populists believed that major corruption existed within political machines, the money interest (banks and corporations), and the business practices of railroads and other industries.
Step-by-step explanation:
The Populists, also known as the People's Party, during the Progressive Era, identified several areas they believed were rife with corruption. They saw political machines like Tammany Hall as corrupt structures that traded jobs and services for votes and maintained power through bribery and graft. Additionally, they saw the money interest, referring to banks and wealthy corporations, as having a corrupt influence on politics due to their control over the money supply, which benefitted them at the expense of the wider public, especially farmers and industrial workers. Finally, the Populists pointed to the business practices of the railroads and other industries, which often involved corrupt interactions with politicians to serve the interests of entrepreneurs over those of the average citizen.