Which of the following developments contributed most significantly to the economic growth of Illinois from approximately 1850 to 1880?

The completion of the Illinois Central Railroad was a critical factor in the economic growth of Illinois during the period from approximately 1850 to 1880. Railroads revolutionized transportation and commerce during the 19th century, and the Illinois Central Railroad was particularly important because it connected the rich agricultural regions of central and southern Illinois to major markets in Chicago and beyond.

This connection facilitated the efficient movement of goods, such as corn and livestock, boosting agricultural production and enabling farmers to gain better access to markets. The railroad also attracted businesses and industries to set up along its route, further stimulating economic development. The presence of the railroad encouraged immigration, as people moved to the region for job opportunities, thereby contributing to population growth and increased demand for goods and services.

Other developments, such as the construction of the Illinois and Michigan Canal and the extension of the National Road, certainly played roles in the state's economic landscape. However, the railroad's capacity for transporting larger quantities of goods over longer distances at greater speeds outstripped the advantages provided by these other transportation methods. Thus, the completion of the Illinois Central Railroad stands out as the most significant contributor to the economic expansion of Illinois during that time frame.