Greyfields are in decaying suburbs.
A empty homes and foreclosed subdivisions
B. sites for future development
C. vacant commercial spaces and empty shopping malls
D. undeveloped land and nature preserves

Greyfields are vacant commercial spaces and empty shopping malls.

Explanation:

Greyfields are basically large objects, usually, ones that were accomodating commercial entities like shopping malls. Because of multiple factors, those shopping malls and other types of commercial entities have closed, mostly because they have been outcompeted by more modern malls that were more attractive to the consumers.

Because they tend to be surrounded by asphalt, and the concrete dominates in the structure, they have been given the name ''grayfields''. Even though they stay unused in the urban landscape, they often offer every condition for a mall to be restarted, as the facilities are there, they tend to have a good location and are well connected with infrastructure.

brave new world, aldous huxley (1932)