Final answer:
U.S. settlers justified taking land from Native Americans under the ideology of Manifest Destiny, which proclaimed their divine right to westward expansion.
Step-by-step explanation:
U.S. settlers felt they had the right to take the Great Plains land from the Native Americans primarily because they believed in the ideology known as Manifest Destiny. This belief asserted that they had the divine right and duty to expand westward, spreading American values and culture. This expansionist mindset often disregarded the existing rights and sovereignties of Native Americans and other groups already inhabiting the lands, legitimizing actions that ultimately led to the forced relocation and subjugation of Indigenous peoples. Therefore, the correct answer to the question is A) They believed in the Manifest Destiny ideology, asserting their divine right to expand westward.