Which is a formal region?
(Points : 1)
The state of Illinois
The Chicago metropolitan area
The historic neighborhoods in Boston
The trade route along the Mississippi River

The State of Illinois is a formal region, with a sharply defined boundary.In principle, with careful enough measurement, you can always find exactly where any place on its boundary is, and you can stand there with one foot inside Illinois and one foot outside of it.  I have done this many times. 

formal means official or recognized so the best answer would be the state of Illinois, the others are general areas