Final answer:
The quote reflects the idea that the American continents should be recognized as independent and free from future colonial rule, correlating with the Monroe Doctrine that opposed European colonialism in the Americas.
Step-by-step explanation:
The quote “The American continents are henceforth not to be considered as subjects” implies that the American continents shall no longer be viewed or used as territories for colonization or control by foreign powers. This reflects the ideology of the Monroe Doctrine, expressing the United States' policy against European colonialism in the Americas following the Latin American countries' independence movements. It essentially meant that the American continents were to be seen as independent entities, free from European intervention and colonial claims, thus endorsing the idea that these continents would neither be new colonial subjects nor be subjected to new forms of colonial exploitation.
When evaluating the options provided, a. The American continents are now independent nations most accurately captures the essence of the statement as it signals the rejection of colonial or external control and the recognition of the political sovereignty of American countries.