Final answer:
The question pertains to natural selection, a fundamental concept in biology, which reveals how certain phenotypes become dominant due to better adaptation to environmental factors like food availability. The best example illustrating this concept is two types of fish with different abilities to gather the same food, indicating a natural selection process.
Step-by-step explanation:
The question concerns the concept of natural selection and its impact on the evolution of species, particularly in relation to the availability of food resources and the fitness of certain phenotypes within populations. When considering the scenarios provided, it becomes evident that situation (b), where two types of fish eat the same kind of food, and one is better able to gather food than the other, provides a clear example of this evolutionary process. It illustrates how individuals with advantageous traits can have increased survival and reproductive success compared to those without such traits, leading to the better-adapted phenotypes becoming more common over successive generations.
In contrast, writing options like (a) which discuss the growth of plants, or options related to seed dispersal do not solely focus on the concept of natural selection. The critical aspect of natural selection is the differential survival and reproduction of organisms due to differences in phenotype, which is highlighted in the fish example. This is significant as it aligns with the observation of Darwin's finches, where different beak sizes evolved due to the selection pressures exerted by the availability of various seed types.