Final answer:
In sexual reproduction, genetic variation is achieved through crossing over, random assortment of chromosomes, and fertilization, but not through asexual reproduction which produces genetically identical offspring.
Step-by-step explanation:
In sexual reproduction, genetic variation is accomplished by all of the following except D. Asexual reproduction. Genetic recombination, which includes processes such as crossing over, random assortment of chromosomes during meiosis, and the random union of gametes during fertilization, are all mechanisms that increase the genetic diversity in sexually reproducing organisms. Asexual reproduction, however, does not contribute to this genetic variation as it involves the production of genetically identical offspring from a single parent without the involvement of gamete fusion.