One of the MOST significant contributions to the agricultural development of the Midwest during the 19th century was —

Establishment of railroad

Explanation:

One of the MOST significant contributions to the agricultural development of the Midwest during the 19th century was the "Establishment of the railroad."

This is evident in the fact that the Midwestern States the which covers areas like Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, etc, is widely known for making agriculture the basis of its economic development.

And with the establishment of Railroad networks, the region was able to distribute its products such as corn and soybeans to other parts of the country.

As of 1910, this region had over 20 railroads network functioning well by linking different states in the country.

both include an amendment process.