Final answer:
Muckrakers were investigative journalists key to the Progressive movement, exposing societal issues and galvanizing public support for reforms.
Step-by-step explanation:
The term muckraker refers to investigative journalists who exposed social, economic, political, and environmental issues during the Progressive movement in the early 20th century. Jacob Riis used photojournalism to bring attention to the poor living conditions in New York tenements. Ida Tarbell is known for her articles exposing the monopolistic practices of Standard Oil. Lincoln Steffens revealed corruption in urban politics, helping to prompt reforms. These muckrakers played a crucial role in highlighting societal problems and mobilizing public support for Progressive Era reforms.