Final answer:
Option a) Manifest Destiny is the correct answer because it is the concept described by John O'Sullivan that encapsulates the belief in the destined expansion of the United States across the continent, which directly supports his claims.
Step-by-step explanation:
The question is asking which of the listed options best supports the claims made by John O'Sullivan regarding the concept of Manifest Destiny. The concept of Manifest Destiny, as expressed by O'Sullivan, encompasses several ideas, including that the United States was predestined to expand its territory across the continent ('from sea to shining sea'), that the expansion would involve the Americanization of new residents, and that this spreading of the United States' boundaries was tantamount to spreading freedom.
Option a) Manifest Destiny directly pertains to O'Sullivan's claims and is therefore the correct answer. The concept was articulated by O'Sullivan himself and indeed supports the idea that the United States was destined for continental expansion with all the ideological implications he attached to it. Neither the Emancipation Proclamation, the Treaty of Versailles, nor the Declaration of Independence specifically relate to O'Sullivan's claims about Manifest Destiny. The Emancipation Proclamation and the Treaty of Versailles are relevant to entirely different historical contexts—namely, the Civil War and the end of World War I, respectively. The Declaration of Independence, while foundational to the United States, predates and does not explicitly concern the mid-19th-century ideology of Manifest Destiny.