Final answer:
Jacob Riis and Upton Sinclair are considered muckrakers because they exposed problems in American society through investigative journalism and writing.
Step-by-step explanation:
Jacob Riis and Upton Sinclair are considered muckrakers because they exposed the problems and injustices in American society through their investigative journalism and writing. Riis used photojournalism to capture the living conditions in working-class tenements, while Sinclair exposed the unsanitary practices in the meatpacking industry in his novel 'The Jungle.' These muckrakers brought attention to social issues and urged for change, ultimately contributing to the Progressive movement in America.