Final answer:
The statement about mutant bacteria surviving antibiotic treatment illustrates D. 'Survival of the Fittest' where only bacteria with resistant genes survive to pass on their traits, thereby increasing the gene's frequency in the population.
Step-by-step explanation:
The statement that occasionally, after a treatment of antibiotic, mutant bacteria may remain and continue to grow illustrates the natural selection factor known as d) Survival of the Fittest. This principle explains that individuals with traits that are favorable for survival in a changing environmental condition, such as the presence of antibiotics, are more likely to survive and reproduce. In this case, bacteria with resistance genes are selected for because the antibiotic wipes out those without the resistance gene. The mutant bacteria that survive, because of their favorable genetic variation that provides resistance to the antibiotic, represent the "fittest" in the context of an environment with antibiotic presence.
This phenomenon also demonstrates that the genetic variation necessary for natural selection was already present within the population before the environmental change, which in this case is the application of antibiotics. Through the process of natural selection, the frequency of the resistance gene increases as the susceptible bacteria are killed off, leaving only the resistant ones to multiply and further propagate their genes.