Built mostly by Chinese and Irish immigrants, it was completed in 1869 at Promontory, Utah, joining the eastern railroad system with California's railroad system at the A)Union Pacific Railroad b) the transcontinental railroad c) the Central Pacífic Railroad d) the Western Pacific railroad

Explanation:

On May 10, 1869, the presidents of the Union Pacific and Central Pacific railroads meet in Promontory, Utah, and drive a ceremonial last spike into a rail line that connects their railroads. This made transcontinental railroad travel possible for the first time in U.S. history. No longer would western-bound travelers need to take the long and dangerous journey by wagon train, and the West would surely lose some of its wild charm with the new connection to the civilized East.