**Core Concept**
Down's syndrome is a genetic disorder caused by an abnormality in the number of chromosomes in an individual's cells. Specifically, it is characterized by the presence of an extra copy of chromosome 21.

**Why the Correct Answer is Right**
The most common cause of Down's syndrome is a nondisjunction event during meiosis, where a pair of chromosomes fails to separate properly, resulting in an egg or sperm cell with an extra copy of chromosome 21. This extra chromosome is present in every cell of the individual, leading to the characteristic features of Down's syndrome. The risk of nondisjunction increases with the age of the mother, particularly over 35 years.

**Why Each Wrong Option is Incorrect**
**Option A:** While trisomy 21 is the most common cause of Down's syndrome, other genetic abnormalities, such as translocations and mosaicism, can also occur. However, these account for a minority of cases.
**Option B:** Mosaicism refers to the presence of both normal and abnormal cells in an individual. While mosaicism can occur in Down's syndrome, it is not the most common cause.
**Option C:** Translocations refer to the transfer of genetic material from one chromosome to another. While translocations can occur in Down's syndrome, they are not the most common cause.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the risk of Down's syndrome increases significantly with the age of the mother, particularly over 35 years. This is because the eggs released by older women are more likely to have undergone nondisjunction during meiosis.

**Correct Answer: D. Nondisjunction during meiosis.**