Development of more railroad systems contributed most to US westward expansion in the second half of the nineteenth century. Option B is correct.
Westward expansion, the 19th-century movement of settlers into the American West, started with the Louisiana Purchase and was driven by the Gold Rush, the Oregon Trail and a belief in "manifest destiny."
In 1862, Congress passed the Pacific Railroad Bill and numerous grants that allowed financial support for the Central Pacific and Union Pacific railroad companies. These two companies then constructed the transcontinental railroad.