The correct option is C It occurs accidentally
Genetic drift can have a drastic effect on smaller populations. It generally reduces genetic variability. Smaller populations are affected more by genetic drift because of sudden loss in the gene pool (sum total of all genes). This results in reduced variation and hence reduced speed of evolution for that species. Genetic drift may occur by pure chance, accident or random events. Genetic drift may or may not offer survival advantage as these changes are not naturally selected. It may even prove to be deleterious for the drifted genes and population.