By 1884, extensive hunting reduced the number of Northern elephant seals to fewer than 100. The extremely limited genetic variation in these seals is consistent with which genetic effect?

Genetic drift, specifically the Bottleneck effect.

Explanation:

Genetic drift is an evolutionary mechanism in which the allelic frequencies in a population change through many generations. Its effects are harder in a small-sized population, meaning that this effect is inversely proportional to the population size. Genetic drift results in some alleles loss, even those that are beneficial for the population, and the fixation of some other alleles by an increase in their frequencies. The final consequence is to randomly fixate one of the alleles. Low-frequency alleles are the most likely to be lost. Genetic drift results in a loss of genetic variability within a population.  

Genetic drift has important effects on a population when this last one reduces its size dramatically because of a disaster -bottleneck effect- or because of a population split -founder effect-.

The bottleneck effect is a special case of genetic drift, which is most likely to affect smaller populations.  In the exposed example, extensive hunting reduced the number of Northern elephant seals, reducing the number of individuals to fewer than 100. This population experienced one or many generations of very small size since these animals were affected by hunting. As the survivors did not have the whole genetic pool of the original population,  the population size might have recovered, but the genetic pool might have not.

b. the carbon cycle ensures living things have a source of hydrogen