The correct usage of "metropolis" in the context provided is:
Chicago, the metropolis of the Midwest, is my favorite city to visit.
This sentence uses "metropolis" appropriately to refer to a major city that serves as a center for a region, aligning with the meaning of "mother city."
The other options do not use "metropolis" in a way that is consistent with its definition as a major city, with some incorrectly implying the meaning of "mother country."