Final answer:
A person with Down syndrome can inherit an extra chromosome 21 due to nondisjunction during meiosis, particularly when maternal age is over 36 as the risk of nondisjunction increases. Therefore, the correct option is B.
Step-by-step explanation:
A person with Down syndrome could have gotten an extra copy of chromosome 21 through a process known as nondisjunction during meiosis. Nondisjunction occurs when the chromosomes fail to separate properly during the cell division that produces sperm or egg cells. As a result, an egg or sperm may end up with an extra chromosome 21, and when it fuses with a normal gamete from the other parent, the resulting zygote will have three copies of chromosome 21, leading to Down syndrome, also known as trisomy 21.
Factors that can increase the risk of nondisjunction include maternal age. The likelihood of nondisjunction events and consequently the risk of having a child with Down syndrome increases in women over 36. However, the age of the male parent is less influential because nondisjunction is much rarer in sperm than in eggs.