In a small group of people living in a remote area, there is a high incidence of "blue skin", a condition that results from a variation in the structure of hemoglobin. All of the "blue-skinned" residents can trace their ancestry to one couple, who were among the original settlers of this region. The unusually high frequency of "blue skin" in the area is an example of .

The unusually high frequency of "blue skin" in the area is an example of genetic drift.

Explanation:

It is a shift in the occurrence of an established gene variant within a population due to spontaneous organism screening. The offspring alleles are a composite of those in the parents and opportunity has a role to play in deciding whether or not a given person persists and reproduces.

The bottleneck impact is an interesting instance of genetic drift which take place when the population size is drastically reduced. Occurrences like natural disasters such as earthquakes, floods, fires can destroy a city, eliminate most people and leave a tiny, random assortment of survivors remaining.

it's c. (to reduce the greenhouse effect/carbon gasses)