Answer:
Natural selection
Step-by-step explanation:
Natural selection is a process where some organisms are able to reproduce more and survive better due to the presence of beneficial genetic traits in them. The other organisms of the same population that lack these traits are not able to survive and reproduce in the same way. The bacterial having drug resistance are able to survive in the presence of drugs in the surroundings. These bacteria will reproduce more and leave more progeny. Over the generations, the proportion of the bacteria with drug resistance will increase. This represents an example of natural selection.