While the practical usage of alternating electric current (Option C) indeed contributed to urban growth by allowing factories to operate away from waterways and facilitating overall industrial growth, the factor that likely contributed the most to the growth of cities in the late 19th century in the United States was the expansion of railroads into western territories (Option B).
The railroads not only connected different parts of the country, facilitating the movement of goods and people, but also encouraged migration to urban areas, spurred economic development, and led to the establishment of new cities and towns along their routes. This infrastructure revolution significantly impacted urbanization during that period.
Therefore, while all factors listed played a role in city growth, the expansion of railroads into western territories was a fundamental driver of urbanization in the late 19th century.