Final answer:
The belief in American cultural and racial superiority contributed to the increased belief in manifest destiny in the early to mid-1800s.
Step-by-step explanation:
During the early to mid-1800s, the increased belief in manifest destiny can be attributed to the philosophy that America was destined to expand its democratic institutions across the country. This belief was underpinned by a sense of American cultural and racial superiority. The assertion that Americans would spread across North America and take economic and political control of the continent incorporated the idea of Americanizing its inhabitants, such as Indians and Mexicans. This notion was often intertwined with the claim that divine providence had chosen Americans to control the Western Hemisphere.
Therefore, the answer to the question is d) The belief in American cultural and racial superiority.