Which of the following early twentieth-century presidents is most closely associated with environmental conservation?

William McKinley, the 25th President, focused primarily on economic issues and foreign policy during his presidency. While he did support some conservation efforts, they were not a central theme of his administration, and he did not implement the sweeping conservation policies that characterized Roosevelt's presidency.

William Howard Taft, although he continued some of Roosevelt's conservation policies, is not primarily known for environmental conservation. His presidency saw the establishment of additional parks and the expansion of national forests, but he lacked the vigorous advocacy for conservation that defined Roosevelt's approach.

Woodrow Wilson's presidency focused on domestic reforms, international relations, and World War I. While he did support some conservation efforts, such as the creation of new national parks, his contributions to environmental conservation were not as profound or impactful as those of Theodore Roosevelt.