In the context of american westward expansion in the nineteenth century, what was the most significant achievement of the lewis and clark expedition?

The peaceful cooperation with the natives.

The most significant achievement of the Lewis and Clark expedition seen in the relationship between the Native Indian and the American settlers. The expedition helped in establishing trade with the Natives and strengthened the rights of the United States in the region. The expedition made an important contribution in discovering new plants and animals like a grizzly bear, swift fox, prairie dog and so on.          

the answer is b.india