Final answer:
The transcontinental railroad was the structure that significantly encouraged westward expansion and economic growth in the United States, linking East and West and facilitating the movement of people, trade, and the spread of American influence.
Step-by-step explanation:
The specific structure that encouraged westward expansion and economic growth in the United States was d) A transcontinental railroad. The construction of the railroad was integral to the nation's expansion.
It linked East and West, enabling the movement of people, facilitating trade, and promoting internal development.
This massive project, completed in 1869, traversed the entire continent, allowing for a more unified nation, improving communication with the use of telegraphs, and providing greater access to the country's vast natural resources.
As part of the broader transportation revolution, the railroad, along with the telegraph, helped to spread American influence and enhance economic development.
These advancements were instrumental in fulfilling the concept of Manifest Destiny, as they propelled the growth of a national market system while also unfortunately accelerating the displacement of Native American populations.