Final answer:
The expansion of the Transcontinental Railroad was the primary factor among the given options that contributed to the U.S.'s industrialization in the late 1800s, as it significantly improved transportation of resources and goods, and catalyzed the growth of industries like steel manufacturing.
Step-by-step explanation:
The industrialization of the U.S. in the late 1800s was significantly propelled by technological advancements and the expansion of infrastructure. Among the options provided, b) The expansion of the Transcontinental Railroad was a critical contributor to the industrial growth during that period. This railway network connected the Eastern U.S. with the Western frontiers, enabling the efficient transportation of raw materials and manufactured goods, as well as facilitating the mass movement of people including immigrants who came to work in growing urban industrial centers. Industries like steel manufacturing, which were vital for building the railroads, thrived as a result. Conversely, options a), c), and d) reference events that occurred earlier and did not have a direct impact on the industrial expansion of the late 1800s.