The correct option is A Sewal wright
The American scientist Sewall Wright (1930) gave the principles of gene frequency fluctuations in small populations in his essay, 'effect of population size'. According to him genetic drift, in small, non-randomly mating populations gene frequencies are found to fluctuate purely by chance. Smaller the population, larger will be the fluctuation in gene frequency. Genetic drift is also called as 'Sewal wright effect'. Thus, option A is correct.