How does natural selection change thephenotypes within a population over time?

Phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. Essentially its physical characteristics. According to natural selection, some characteristics are more likely to survive and thus pass on to children of a generation. Therefore, over time said characteristics such as the ability for an organism to blend in with its environment get passed along and become more prominent within a population ultimately making the appearance (or phenotype) of the population more synonymous with the characteristics "chosen" by natural selection.    

Explanation:

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."