The answer is "d. prospectively; nonconforming".
Zoning laws act "prospectively"; that is, uses and buildings that already exist in the zoned area are permitted to continue even though they do not fit within new zoning ordinances. such uses are called "nonconforming" uses.
The essential reason and capacity of zoning is to isolate a region into private, business, and mechanical regions (or zones), that are generally separate from each other, with the utilization of property inside each region being sensibly uniform.
