The correct option is D Interbreeding within small population
Genetic drift is concerned with the gene frequency of a reproducing small population. In a small population not all the alleles which are representatives of that species may be present. In such a small population a chance event may increase the frequency of a character that has little adaptive value. Thus, genetic drift may remain a significant factor in the origin of new species on islands and other isolated populations. The small population as a whole may develop characters different from that found in the main population. Such deviations may even lead to speciation or formation of a new species. The interbreeding of the population may result in the development of the new species.
So, the correct answer is- Interbreeding within small population