Options A-D was the subject of a muckraker’s investigation. This is further explained below.
Generally, During the Progressive Era (1890–1920), investigative reporters and writers were known as "muckrakers." These individuals wrote about social issues such as corruption and injustice in an effort to bring about improvements in society.
In conclusion, the business practices of the railroads, Rockefeller’s Standard Oil monopoly, and others were the subject of a muckraker’s investigation.
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