The correct answer is B.
Genetic drift is one of the four evolutionary mechanisms. It changes the frequency by chance, aka at random.
For example. if you have 100 frogs in the population, 20 red, 20 green and 60 yellow, not all of the alleles are going to be passed down to the next generation because a portion of the population can be affected by a random event such as the flood. So, if the flood accidentally kills 20 red frogs, none of the offspring are going to be red by chance.
