The first westward surge of settlement reached the:

pacific ocean
rocky mountains
mississippi river
appalachian mountains

The Appalachian Mountains

The American expansion took place between the early 17th century and ended with the admission of the last mainland territories as states in 1912. Most of it consisted on the acquisition of Indigenous lands in what is now the Midwest, Texas, the Great Plains, the Rocky Mountains, the Southwest and the West Coast. This expansion was particularly encouraged through the idea of "Manifest Destiny."

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