The correct answer is C) the growth of the railroads increased the demand for natural resources.
The growth of the railroads affected the supply and demand for natural resources in the United States in that the growth of the railroads increased the demand for natural resources.
Railroad transportation changed the way American people traded. For instance, agriculture products were transported easily throughout the country. Meatpacking industry also benefited a lot. Natural resources such as coal became more available in all the country due to the railroad expansion. Railroad increased the demand for natural resources because railroads facilitated its transportation. The Steel industry also benefited with the railroads.
The other options of the question were, a) decreased the supply of natural resources, b) decreased the demand for natural resources, and d) had no effect on the supply of natural resources.