Who investigated and reported John D.Rockefeller's corrupt business practices at Standard oil?

The person who investigated and reported John D. Rockefeller's corrupt business practices at Standard Oil was Ida Tarbell.

Ida Minerva Tarbell was an American professor, writer and journalist, considered one of the main "muckrakers" of the Progressive Era of the late 19th and early 20th century and a pioneer of investigative journalism.