Final answer:
Therefore, the correct option for the question, 'Natural selection results in change over time by acting on traits that are:', is D. Heritable. Examples like the long necks of giraffes illustrate that it's the heritable traits which can be passed from generation to generation that are subject to natural selection.
Step-by-step explanation:
Natural selection results in change over time by acting on heritable traits that organisms possess. Variations that arise due to gene mutations or gene flow become part of the gene pool and come under the influence of natural selection. For a trait to be acted upon by natural selection, it must be capable of being passed down through generations; this means it must be part of an individual's genetic makeup.
Such traits, if beneficial in certain environments, enhance an organism's chances of surviving and reproducing, thereby increasing the frequency of these advantageous alleles in the population. Conversely, detrimental heritable traits tend to diminish in frequency over generations as they confer a survival or reproductive disadvantage.
For example, consider a population of giraffes where variation occurs and some giraffes have longer necks due to genetic variation. In an environment where the food resources are higher up in trees, having a long neck is beneficial because it allows giraffes to access more food. Over time, natural selection would favor these long-necked giraffes, and we would expect to see the trait of a long neck become more prevalent in the giraffe population.
Therefore, the correct option for the question, 'Natural selection results in change over time by acting on traits that are:', is D. Heritable.