The correct answer is to prevent European intervention in the Western hemisphere.
The Monroe Doctrine (1823) was created by president James Monroe. The goal of this ideology was to stop further European colonziation in the western hemisphere, specifically in the Latin American countries. The US established this policy to assert their control over the Latin American countries and also to develop a policy of isolationism in terms of European affairs.
The Roosevelt corollary built upon this idea, stating that the US would serve as "international police" in the sense that they will support Latin American countries against unjust invasion/use of force by European countries.