Crossing over is the process of exchange of genetic material between non-sister chromatids of homologous chromosomes to produce new genetics combinations or variations; it does not interfere with independent assortment of genes. Recombination is new arrangements of genes on chromosomes which is carried out by independent assortment and crossing over; thus recombination is result of independent assortment. Repulsion is the tendency of genes present on different chromosomes to stay separate. Linkage is referred to the phenomenon wherein alleles located close together on a chromosome show tendency to be inherited together during meiosis. Genes present near to each other have less probability of being separated onto different chromatids during chromosomal crossover. Therefore, lack of independent assortment of two genes A and B in fruit fly Drosophila is due to linkage. Hence, the correct option is D.