The correct answer is: "declared that European nations must not colonize nations in the Western Hemisphere"
The Monroe Doctrine was a declaration that comprised the intentions of the US in foreign policy at the moment. It was issued in 1823 as a mechanism of opposition to European colonialism. In that moment, most former Latin American Spanish and Portuguese colonies had gained independence of were about to do it.
The declaration stated that any actions unertaken by European countries aiming to exercise dominance over any American territories would be regarded by the US "as the manifestation of an unfriendly disposition towards the United States", that would act in consequence.