Final answer:
Advanced maternal age is the risk factor for the development of Down's syndrome. This increased risk is due to the higher frequency of chromosomal nondisjunction during meiosis in older women, leading to trisomy 21.
Step-by-step explanation:
The correct answer to the question of which risk factor is associated with the development of Down's syndrome is C) Advanced maternal age. Down syndrome, also known as trisomy 21, is caused by the presence of an extra copy of chromosome 21. The most common cause of Down syndrome is chromosomal nondisjunction during meiosis, an error in cell division where chromosomes fail to separate properly. Evidence indicates that the frequency of nondisjunction events, and therefore the risk of having a child with Down syndrome, increases with maternal age.
The incidence of having a fetus with trisomy 21 increases dramatically in mothers over the age of 35. The age of the mother is a significant risk factor, while the age of the father plays a smaller role because nondisjunction is much less common in sperm than in eggs.