Answer : The correct answer is -
A-meiosis.
Meiosis is a type of cell division (a reductional cell division) that occurs during the process of formation of gametes particularly in sexually reproducing organisms.
During prophase of meiosis I, crossing over between homologous chromosomes takes place, resulting in the genetic diversity in new chromosome that come in the gametes.
Apart from this, when gametes from two parents fuse, new genetic combination is produced in the resulting zygote.
Thus, meiosis is a source of genetic variation in case of sexually reproducing organisms.