The correct answer is c: the land had many natural resources and few settlers.
During the 1820s, American expansionists were motivated by the potential resources and opportunities available in these territories, which were seen as favorable for agriculture, commerce, and settlement. Manifest Destiny, the belief that the expansion of the United States across the American continents was both justified and inevitable, became more prominent in later years, but the primary motivations of the 1820s revolved more around land and resources.