Final answer:
During the era of railroad expansion, museums primarily focused on educating the public about C) Industrial innovations, highlighting the transformative impact of railroads and the growth of corporate America.
Step-by-step explanation:
The aspect of American business that museums primarily focused on to educate the public during the era of railroad expansion was C) Industrial innovations. This period saw significant growth in industrial development, driven by entrepreneurs like Andrew Carnegie and John D. Rockefeller. Railroads played a crucial role in the expansion across the United States, connecting resources to manufacturing locations and spreading the 'American' way of life as espoused by the eastern establishment and Manifest Destiny.
As the railroad networks expanded, they enabled the growth of cities and the economy, supporting large-scale agricultural practices and fostering transitions from individualistic ventures to big business. Institutions like museums reflected these changes by showcasing the new technologies and industrial advancements that were shaping America's economy and society.
Railroads, corporate growth, and the transformation of the American landscape were all aspects depicted in exhibits and educational programs of the time, helping individuals to broaden their understanding beyond their immediate experiences.