(mc)which group worked to reduce the power of political machines during the late 19th and early 20th century

The answer is progressive reformers.  A political machine is a political organization in which a commanding boss or small group instructs the support of a corps of supporters and businesses who receive rewards for their efforts. Political reforms presented the primary indications for a progressive movement and for sustained reform through the presidencies of Theodore Roosevelt and Woodrow Wilson. And because of the progressive reformers, it limited the power of political machines through the Pendleton Civil Service Act. It is a set up a merit system for awarding federal jobs to help eradicate giving jobs to inexpert workers as spoils of elections; this limited the elected officials from hiring and firing candidates based on their political commitments.