I believe the answer is: D - the exchange of genetic material between homologous chromosomes.
Step-by-step explanation:
In crossing over, sections of DNA move between homologous chromosomes and allows for independent assortment.
Independent assortment states that genes are inherited independently of one another.
For meiosis to occur, the chromosomes contributed by each of the organism's parents are duplicated to form sister chromatids. During meiosis I, the sister chromatids of one parent match up with the corresponding sister chromatids of the other parent (synapsis), or its homologous non-sister chromosome, along the metaphase plate.
Crossing over then occurs. At a point called a chiasma, homologous chromosomes trade genetic information so that each chromosome is complete but has different information.
This random exchange of information is what allows for unique gametes to form and genetic recombination to occur.