The importance of the Monroe Doctrine to European countries was established in the United States as what?

Although initially disregarded by the great powers of Europe, the Monroe Doctrine became a mainstay of U.S. foreign policy. In 1823 U.S. President James Monroe proclaimed the U.S. protector of the Western Hemisphere by forbidding European powers from colonizing additional territories in the Americas.

i beleive that d is the right answer