Final answer:
Antibiotics select for bacteria with pre-existing resistance genes, an example of evolution through natural selection. Resistant bacteria survive antibiotic treatment, proliferate, and increase the frequency of resistance in the bacterial population.
Step-by-step explanation:
The correct answer to how antibiotics affect a population of bacteria is d) Antibiotics select for antibiotic-resistant bacteria. When antibiotics are applied to a population of bacteria, those with existing resistant traits survive, as the antibiotic kills non-resistant bacteria. This is an example of evolution through natural selection, not because the antibiotics induced mutations, but because the resistant bacteria had a pre-existing genetic advantage that allowed them to survive. Due to this selective pressure, the resistant bacteria reproduce, and the proportion of resistant bacteria increases in the population.
The excessive use of antibiotics contributes to this problem by increasing the selection pressure on bacteria, leading to more of these organisms being antibiotic-resistant. It's also worth noting that the overuse of antibiotics in livestock furthers this issue by promoting antibiotic resistance, which can then be transferred to human populations.