The answer is C. Staphylococcus aureus that had a mutation allowed them to synthesize cells walls using a protein that was not affected by methicillin and these bacteria survived the treatment of that antibiotic and were able to reproduce at higher rates. This was a pre-adaptation, something in the DNA of some individuals that was due to a variation or mutation in their DNA. Once this antibiotic was introduced into their environment, those with the mutation had a selective advantage and survived. The selecting agent wasn't "nature" rather, it was the antibiotic. Those with the favorable mutation survived and because they were fittest, reproduced and passed along this variation to offspring. Also, because bacteria are able to conjugate, they exchanged their DNA with other individuals, causing the resistant gene to spread through the bacteria population.