Which of the following best explains why certain traits become more common in a population over generations?

Natural selection is the process by which certain traits become more common in a population over generations due to those traits providing a survival or reproductive advantage. In any given environment, individuals with traits that enhance their ability to survive and reproduce are more likely to pass those advantageous traits on to the next generation. This results in an increase in the frequency of those traits within the population over time.

For example, if a particular trait allows organisms to better evade predators or find more food, those individuals are more likely to survive long enough to reproduce. This process leads to the gradual evolution of the population, favoring traits that are beneficial for survival and reproduction in the specific environment.

Other processes listed, such as genetic drift, gene flow, and environmental adaptation, do not primarily explain the increase of certain traits through selective pressure in the same way natural selection does. Genetic drift refers to random changes in allele frequencies that can occur in small populations, while gene flow involves the movement of alleles between populations and can introduce new genetic material, but it does not necessarily favor particular traits. Environmental adaptation is a broader term and relates to how organisms adjust to their environment but doesn't directly address the mechanism of trait frequency change over generations as effectively as natural selection does.