Final answer:
The idea described in the quotation from John L. O'Sullivan's 'The Great Nation of Futurity' is a) manifest destiny.
Step-by-step explanation:
The idea described in the quotation from John L. O'Sullivan's "The Great Nation of Futurity" is manifest destiny. O'Sullivan expresses the belief that the United States is destined to expand its territory and spread its ideals of freedom and equality.
This concept of manifest destiny asserts that America has a divine mission to bring democracy and American culture to the entire continent.
O'Sullivan's quote emphasizes the inevitability and righteousness of this destiny, portraying it as an eternal decree of cause and effect.