Answer:
The correct answer would be C) genetic drift.
Genetic drift is defined as the change in the relative frequency of alleles in a population as a result of a sudden decrease in the population size.
Events like natural disaster, excessive hunting et cetera can drastically reduce the population size in a very short period of time.
It results in the loss of many alleles and thus the genetic variation of the population gets lost.
Similarly, hunting reduced the population of northern elephant seals to as low as 20 individuals due to which most of the genetic diversity of the population was lost.