In the late eighteenth century, a typhoon swept through the Pacific atoll of Pingelap, leaving approximately 20 survivors. A large percentage of the present-day inhabitants of Pingelap are color blind. One can conclude, therefore, that the population experienced a .

Genetic bottle neck

Explanation:

Genetic bottle neck is defined as an evolutionary event during which a population is drastically reduced to to a significantly smaller size within a short period of time.

A genetic bottleneck can result from various events such as natural disaster, hunting of a species to a point of extinction or habitat destruction to cause the death of large number of organism.

In the given situation a typhoon which is a type of natural disaster caused the population to reduce to a size of  20 individuals. Then we find that the allele frequencies were greatly changed  and the allele for colorblindness became established.Hence, a large population of present day inhabitant are color blind.

So we can say that the population experienced a genetic bottle neck.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

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