Appearance of ant A. adaptive radiation

B. transduction

C. pre-existing variation in the population

D. divergent evolution

Adaptive radiation – This is evolution of different species for a single organ depending on the geographical area in which they live.

Transduction – This refers to transfer of different viral or bacterial genetic materials.

Divergent evolution – This refers to homologous organs which are similar anatomically but perform different functions.

The appearance of antibiotic-resistant bacteria is an example of pre-existing variation in the population. So when a bacterial population encounters a particular antibiotic, those sensitive to it die. But some bacteria having mutations become resistant to the antibiotic. Soon, the resistance providing genes become widespread and the entire population becomes resistant.

Hence the correct answer is option (c).