Final answer:
Manifest Destiny was the 19th-century doctrine that the United States had the right to expand its territory across North America, Americanizing the inhabitants and spreading democratic institutions.
Step-by-step explanation:
Manifest Destiny was the idea that America had a certain right to expand its landholdings. The concept emerged in the 19th century and justified in Americans' minds their right and duty to govern any other groups they encountered during their expansion. It also absolved them of any questionable tactics they employed in the process. Although sometimes connected with the spread of Christian religion or other values, the core of Manifest Destiny was the expansion of the United States territory "from sea to shining sea."
The belief was that the destiny of the United States was to spread democratic institutions across the continental United States. This drive for expansion would inevitably lead to the Americanization of indigenous peoples and residents of territories such as Texas, Oregon, and California and was often justified as the spreading of freedom. The phrase itself was first employed by magazine editor John O'Sullivan, who wrote that it was the nation's manifest destiny to overspread and possess the whole of the continent which Providence had given for the development of the great experiment of liberty.
In summary, Manifest Destiny encompassed ideas that America was to extend its boundaries across North America, Americanize its residents, and spread its democratic values, thus the correct option for this question is that Manifest Destiny was the idea that America had a certain right to expand its landholdings.