What was the purpose of the Monroe Doctrine?
O It was a treaty for the United States to buy land.
O It increased trade with other nations in Europe and Asia.
It warned European nations to stay away from Western nations.
It was a policy to increase the size of the U. S. military.

Right choice is:

It warned European nations to stay away from Western nations.

Explanation:

The Monroe Doctrine is a concept that sees any intervention of extrarregional powers (mainly of Europe in 1823) in the affairs of the nations of the Western Hemisphere as a threat to the United States´ security. It also states that the US would not intervene in the conflicts or issues of the European powers.

to extend a country’s power and influence