Vast swathes of suburbia have turned into a ____________: homeowners, unable to pay their mortgages, have simply left their domiciles, creating a veritable graveyard of empty houses.

What comes after the colon define/describes/explains what comes before it. Thus description of the what the suburbia has become(turned into) is given in the following sentence fragment:

homeowners, unable to pay their mortgages, have simply left their domiciles, creating a veritable graveyard of empty houses.

The word from the choice list is necropolis.