In the mid-1800s, Native Americans who had lived in the West A. traded their manufactured goods with settlers.
B. were able to resist diseases carried by settlers.
C. offered their services as guides to settlers.
D. prevented settlers from moving onto their territory.

I know it's not A.

The correct answer is C) offered its services as guides to settlers.
In the mid-1800s, American Indians that had to live in the West offered its services as guides to settlers.
In those years, the Indian tribes that inhabited the Western territories of America saw in settlers the opportunity to offer their services as guides because they knew very well the geography of those territories. This helped in the increase of settlers in the region because in the early 1800s only 10% of colonizers lived West the Appalachians Mountains. But by 1825, 30% of people lived between the Appalachians and the Mississippi River. American Indians guiding serve people to guide them through these new territories.