In the karyotype of the question, we can see that there's a trisomy 21, which means that there're three chromosomes (D. Trisomy 21), instead of two in the 21 position of the sheet. This causes a disease known as Down syndrome, and it's caused because instead of inheriting 23 chromosomes from each of our parents, there would be 24 and 23 chromosomes, having 46 pairs plus one extra chromosome.