3. What is Manifest Destiny?

Manifest Destiny is the idea that the United States is destined by God to expand its region and spread democracy and capitalism across the North American continent.

the 19th-century doctrine or belief that the expansion of the US throughout the American continents was both justified and inevitable.

Explanation:

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain