Final answer:
The statement that best describes the role of railroads in the United States' industrialization is that they provided an efficient means of transportation during the Age of Big Business, which contributed significantly to the country's economic development and the growth of other industries.
Step-by-step explanation:
The statement that best describes the role of railroads in the industrialization of the United States is: A. They provided an efficient means of transportation during the Age of Big Business. The development and expansion of the railroad system were integral to the transportation revolution in the 19th century.
Railroads not only facilitated the movement of raw materials to mills and factories but also enabled the distribution of finished goods to markets far and wide. Railroad companies emerged as the first 'big businesses' of the nation, providing inexpensive and consistent transportation which in turn stimulated growth in industries such as coal, iron, and steel.