**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 occurs when there is an error in cell division during meiosis, resulting in an abnormal number of chromosomes in the offspring.

**Why the Correct Answer is Right**
The correct answer is 21 trisomy because Down's syndrome is characterized by the presence of an extra copy of chromosome 21, leading to an abnormal number of genes. This extra genetic material disrupts normal cellular function, resulting in the characteristic physical and intellectual disabilities associated with Down's syndrome. The extra chromosome 21 is the result of a nondisjunction event during meiosis, which can occur in either the sperm or egg cell.

**Why Each Wrong Option is Incorrect**
* **Option A:** A 5/12 translocation is not associated with Down's syndrome. Translocations involve the exchange of genetic material between chromosomes, but this specific translocation is not related to the disorder.
* **Option B:** 18 trisomy is associated with Edwards syndrome, a different genetic disorder. While both conditions involve an extra chromosome, the affected chromosomes are different.
* **Option D:** 14/21 translocation is a type of chromosomal abnormality, but it is not the primary cause of Down's syndrome. This translocation involves the exchange of genetic material between chromosomes 14 and 21, but it is not as common as trisomy 21.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Down's syndrome can also occur due to mosaicism, where only some cells have the extra chromosome 21. This can make diagnosis more challenging, and it is crucial to consider this possibility in cases of suspected Down's syndrome.

**✓ Correct Answer: C. 21trisomy**