Final answer:
The correct answer is c) Steel, which was crucial for railroad construction, buildings, bridges, barbed wire, and skyscrapers during America's industrial expansion.
Step-by-step explanation:
Toward the end of the 19th century, steel became a fundamental material in American industrial growth, used extensively in railroad construction, the development of skyscrapers, and structural frameworks for buildings and bridges. The transcontinental railroad, in particular, advanced the steel industry significantly, creating a demand for the robust material. Improvements in steel manufacturing also saw its use in barbed wire, facilitating westward expansion. The answer to the student's question is c) Steel, which was utilized in the construction of railroads, buildings, bridges, barbed wire, and skyscrapers during the period of rapid industrial expansion.