Final answer:
Darwin believed that traits contribute to evolution if they provide either a reproductive or survival advantage (option d). This concept is known as natural selection, where traits that improve survival or reproductive success are more likely to be passed down and become prevalent in a population.
Step-by-step explanation:
Darwin believed that a trait contributed to evolution by providing either a reproductive or survival advantage. This concept is central to his theory of natural selection.
In Darwin's view, the process of natural selection means that individuals with traits that give them an advantage in survival or reproduction are more likely to pass on these traits to their offspring. Over time, such advantageous traits become more common within a population because individuals that are better adapted to their environment tend to survive and reproduce more successfully than those who are not as well adapted.
The variation in traits, such as the beaks of finches that Darwin observed, demonstrates how adaptation to the environment can gradually occur. This ultimately leads to a population that fits better with its surroundings, which is the essence of adaptive evolution.