Which u. s. region's natural resources made it a key area for the production of iron and steel in the late 1800s?

That would be the Midwest.

The Midwest in the 1800s and all the way to the 20th century was an important center in the United States for manufacturing, especially the steel. The region was full of natural resources while it was also supremely well connected with the rest of the country which allowed goods to travel more freely and more swiftly. There was also a lot of fuel for the factories there so it made sense for them to be situated there.