In the mid-1800s, pioneers who traveled through the great plains to the west faced
a. summer rainstorms and difficult terrain.
b. summer rainstorms and rugged mountains.
c. steep passes and difficult terrain.

Generally speaking, in the mid-1800s, pioneers who traveled through the Great Plains to the West faced "c. steep passes and difficult terrain" since this territory was wildly unpredictable and oftentimes dangerous. 

They were eliminating problems caused by industrialization, urbanization, immigration, and corruption in government