Final answer:
Option (A), The transcontinental railroad and the Homestead Act expedited the westward expansion of the United States by providing efficient transportation and land incentives, which led to the inclusion of new states in the Union.
Step-by-step explanation:
The transcontinental railroad and the Homestead Act profoundly impacted the political boundaries of the United States by leading to vast Western expansion and the addition of new states. This expansion was facilitated by the creation of the transcontinental railroad which, upon its completion in 1869, made it easier for settlers to travel and for goods to be transported across the continent.
The Homestead Act of 1862 provided incentives for Northern free-labor farmers and others to relocate to the West by offering 160 acres of federal land to individuals who would reside on and improve the property for five years, thus promoting agricultural development and settlement.
These legislative actions not only encouraged the displacement and decimation of Native American tribes, who were forced onto reservations, but also resulted in the incorporation of newly established territories into the Union as states, demonstrating a tangible expansion of the United States westward, with states such as Nebraska, Colorado, North Dakota, South Dakota, Montana, Washington, Idaho, Wyoming, Utah, Oklahoma, New Mexico, and Arizona joining the Union between 1867 and 1912.