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.