Genetic variation in a population does not take place due to

The correct option is C Crossing over between homologous chromosomes during meiosis II
In sexually reproducing organisms, meiosis produces haploid gametes which later on fuse to form the diploid organism. Thus, there is a regular alternation between haploid gametes and diploid adult, this is possible due to meiosis. Meiosis involves the pairing of homologous chromosomes, crossing over between homologous chromosomes leading to exchange of segments between non-sister chromatids of a homologous pair of the chromosome which takes place between prophase I and metaphase I of meiosis I. In sexually reproducing organisms, meiosis produces haploid gametes which later on they fuse to form the diploid organism. Thus, there is a regular alternation between haploid gametes and diploid adult, this is possible due to meiosis. Random mutation is the greatest source of mutation as it alters the gene flow. Thus the correct answer is option D