A small factory has been operating in an area for $15$ years. The city recently rezoned the area as strictly residential. What is likely to happen to the factory?

$($a$)$ It must cease operations immediately.

$($b$)$ It can continue only if it obtains a special use permit.

$($c$)$ It must convert to a residential use within a specified time frame.

$($d$)$ It can continue operating as a legal nonconforming use.