Final answer:
Manifest Destiny was the 1840s belief that justified white settlers' westward expansion onto indigenous lands, promoting the idea that American expansion was inevitable, just, and divinely ordained. Option a is the correct answer.
Step-by-step explanation:
The belief or event in the 1840s that justified the westward movement by white settlers onto indigenous lands was Manifest Destiny. The concept of Manifest Destiny arose as a religious and cultural justification for American expansion across the continental United States. It was based on the belief that the United States was destined to spread democratic institutions "from sea to shining sea."
This belief posited that Americans were meant to expand to the limits of North America, taking political and economic control of the continent. As a result, supposedly, any person or community that was encountered, including Native Americans and Mexicans, would be Americanized, and any resistance would be forcibly quelled. Some proponents suggested that God had chosen Americans for this role, a view that helped push westward expansion, often at the expense of other peoples and nations.