Final answer:
Option (a), The primary reason for U.S. settlers heading west in 'Story I: Virtual' was economic opportunities and land availability, inspired by the notion of Manifest Destiny.
Step-by-step explanation:
In "Story I: Virtual," the most important reason for U.S. settlers heading west was a) Economic opportunities and land availability. Driven by the ideal of Manifest Destiny, settlers were attracted by the promises of ample land for farming, cattle, and gold. The lure of economic prosperity was significant, and the word of mouth about the fertile lands beyond the Mississippi fueled their desires.
Furthermore, the discovery of gold in California and the availability of cheap land in the west enticed many. While political freedom, the quest for adventure, and the Civil War were also factors, the primary motivator was the economic opportunity that the vast and seemingly endless western lands offered.