An edge city is a that develops the qualities and services usually found in central cities.
a. greyfield
b. declining suburb
c. tract housing development
d. growing suburb

An edge city is a growing suburb that develops the qualities and services usually found in central cities. It's a type of area in the city that's starting to develop further just like its central locations, and once developed, can be considered as a part of the central city - with new edge cities under development.