c) Rocky Mountains

The Rocky Mountains posed a major natural barrier for pioneers traveling west. This mountain range, extending from the northernmost part of British Columbia in Canada to New Mexico in the U.S., presented significant challenges due to its rugged terrain, altitude, and harsh weather conditions. Overcoming this obstacle was a formidable task for settlers moving westward during the 19th century. The successful navigation through passes such as South Pass in Wyoming was crucial for access to the Oregon Territory and California, playing a vital role in the westward expansion of the United States.