**Core Concept**
Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This extra genetic material alters the development of the brain and body, leading to intellectual disability and varying degrees of physical abnormalities.

**Why the Correct Answer is Right**
The correct mutation in Down's syndrome is a trisomy of chromosome 21. This occurs when there is an error in the meiotic division of the gametes, resulting in an egg or sperm that contains an extra copy of chromosome 21. When this abnormal gamete is fertilized, the resulting zygote has 47 chromosomes instead of the usual 46, leading to Down's syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to Down's syndrome.
* **Option B:** This option refers to a different genetic disorder, not Down's syndrome.
* **Option C:** This option is incorrect because it does not describe the genetic mutation seen in Down's syndrome.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Down's syndrome is the most common chromosomal disorder, and the risk of having a child with Down's syndrome increases with the mother's age. Women over 35 years old are at a higher risk of having a child with Down's syndrome.

**Correct Answer: C. Trisomy 21.**