Answer: The correct answer is A) Nondisjunction
Down syndrome is a chromosomal disorder that is also termed as Trisomy 21 that is there is an extra copy of chromosome 21.
It is caused due to meiotic non disjunction ( improper separation of chromosomes) where one gamete receives one extra copy of chromosome 21 that enters in the zygote when two gametes ( one from each parent) combines together making the cell trisomic that is (2n+1)
Thus, nondisjunction is the right answer.
