**Core Concept**
Down's syndrome, also known as trisomy 21, is a genetic disorder caused by an abnormality in the number of chromosomes. It results from the presence of an extra copy of chromosome 21, leading to developmental and intellectual disabilities.

**Why the Correct Answer is Right**
The most common cause of Down's syndrome is nondisjunction, which occurs during meiosis when a pair of chromosomes fails to separate properly, resulting in an extra copy of chromosome 21. This can happen during either maternal or paternal meiosis, but it is more common in older mothers due to the increased frequency of meiotic errors. The presence of an extra chromosome 21 disrupts the balance of genetic material, leading to the characteristic features of Down's syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mosaicism is a rare cause of Down's syndrome, accounting for about 1% of cases. It occurs when a mixture of cells with 46 and 47 chromosomes is present.
* **Option B:** Translocation accounts for about 4% of Down's syndrome cases. It occurs when a part of chromosome 21 breaks off and attaches to another chromosome, usually chromosome 14.
* **Option D:** Mosaicism and translocation are specific mechanisms that can cause Down's syndrome, but they are not the most common causes.

**Clinical Pearl / High-Yield Fact**
Older maternal age is a significant risk factor for Down's syndrome, with the risk increasing significantly after the age of 35.

**Correct Answer: A. Nondisjunction.**