The correct answer is D) California.
The state that was most important to the fulfillment of the dream of manifested destiny was California.
In January 1948, James Wilson Marshall discovered gold along the American River in the North part of Sacramento, California. When word spread to the East, people sold their things in order to travel to California pursuing the American Dream. The California Gold Rush had started. So it is correct to say that was most important to the fulfillment of the dream of manifested destiny was California.