Which of the following is an example of adaptation?

a. a city-dwelling mouse population that on average becomes better at digesting discarded food over several
b. a seabird population in which all of the members change feather color from white to gray as the seasons change
c. a deer population where adult deer develop immunity to a virus after several years of exposure
d. a colony of bacteria that activates antibiotic resistance genes when exposed to an antibiotic