The correct answer is he broke up the Northern Securities Company.
Theodore Roosevelt was the "trust-buster" that is he broke up monopolies and the Northern Securities Company was a railroad trust that consisted of E. H. Harriman, James J. Hill, J.P. Morgan and of course those who worked with them. They were the first company that was sued in 1902 under Sherman Antitrust Act which was enforced by Justice Department who were working under orders by Theodore Roosevelt.