Final answer:
John L. O'Sullivan believed expansion was necessary to a. achieve America's destiny, a concept linked with Manifest Destiny which equated territorial expansion with the spreading of freedom and democratic ideals.
Step-by-step explanation:
John L. O'Sullivan and others like him believed that expansion was necessary to achieve America's destiny. This concept is tied to the historical idea of Manifest Destiny, which held that the United States was divinely ordained to expand democratic institutions across North America, from sea to shining sea. In the process, inhabitants of the continent would be Americanized, and the notion of spreading the U.S. boundaries was equated with the spreading of freedom. The expansion movement, while economically and politically motivated, also involved complex questions about the morality of expansion, the nature of freedom, republicanism, and the effects of acquisition of new territories on national security and economic interests.