Final answer:
The correct answer is d. Both preserving favorable variations and rejecting harmful variations in a population, as natural selection acts on these traits to enhance survival and reproduction over time.
Step-by-step explanation:
According to Darwin, natural selection usually preserves favorable variations in a population and rejects harmful variations. Therefore, the correct answer to the question is d. Both A and B are correct. Natural selection reproduces individuals with favorable genetic traits, such as the adaptation of better running speed, over time.
This process selects for individuals with traits that increase their chances of survival and reproduction, also known as Darwinian fitness, while simultaneously selecting against those with detrimental traits. Examples of this can be seen in numerous species adapting to their environments over generations.