Answer:
Option A, Genetic drift
Step-by-step explanation:
Options for the question
a.genetic drift
b. gene flow
c. increased rate of spontaneous mutations
d. adaptive radiation
Solution
Genetic drift is an evolutionary mechanism in which the random fluctuations occur in the gene pool of a given population. In general, this variation of allele frequency continues till the allele is lost or it persists as the only allele in the population.
Due to such changes in the allele population, the genetic diversity reduces as is the case with the cross river gorilla whose gene pool has reduced due to fluctuations such as poaching and Ebola disease.
Hence , option A is correct