The correct option is D. Taft’s inability to successfully implement Roosevelt's policies.
President Theodore Roosevelt mentored William Howard Taft to be his successor as President in the White House. However, once Taft was elected in 1908, he based his administration on a more business-centric and limited-government, in direct conflict with Roosevelt’s progressive vision. Roosevelt, displeased with his former disciple’s approach, ran for president in 1912 to unseat Taft and, but when he couldn’t build significant support among Republicans he formed the Progressive Party. Such split in Republicans resulted in the election of the Democratic contender, Woodrow Wilson.