The correct answer is: A. the work of the Progressive Movement.
Indeed, the Progressive Era came as a reaction to the extreme abuses of the Gilded Age and the multimillionaire robber barons of the second half of the 19th century. Social, racial, political and gender inequality were rampant in the United States and several civil organizations sought to not only mitigate the effect of such situations but to reverse them and change society for the cause of common welfare.
One of the main reasons that wealthy individuals wielded so much power over society was their grip on government at all levels. They funded and bribed politicians into manufacturing laws that benefited them and not only went against the interests and welfare of the majority of the population; these laws also criminalized all kinds of dissent.
Progressives strove to have states’ constitutions amended to include the legal right to organize referendums in order to recall corrupt government officials. There was also the issue of US Senators, before 1913, they were not elected by popular vote but by appointment by the state legislatures. This meant that in many cases, corrupt state legislatures elected corrupt Senators that gridlocked all the lawmaking processes in favor of the private, wealthy interests they represented.