Answer;
B. When an antibiotic is used, bacteria that have a natural variation for resistance to the antibiotic have a greater chance of survival.
Step-by-step explanation;
Antibiotic resistance is a consequence of evolution via natural selection.
The antibiotic action is an example of environmental pressure to bacteria, therefore; those bacteria which have a mutation allowing them to survive will live on to reproduce.
They will then pass this trait to their offspring, which will be a fully resistant generation.