The right answer is A. genetic drift
From the point of view of a gene, genetic drift leads to the increase or decrease of the frequency in the population, of one of its versions (= allele).
In any population, it is statistically inevitable that some alleles (each variant of the same gene) are not transmitted by any adult to their offspring. In addition, some individuals have no descendants at all. The number of alleles (genetic variability) is therefore reduced. Among the "surviving" alleles, some will see their original frequency decrease or increase, all without implying the appearance or the addition of new alleles.
Two concrete examples are a foundation effect and a decrease in genetic diversity.