**Core Concept**
Down syndrome is a genetic disorder caused by an abnormality in the number of chromosomes, specifically trisomy of chromosome 21. This results in the presence of an extra copy of the genes on chromosome 21, leading to various developmental and intellectual disabilities. The condition is often referred to as Trisomy 21.

**Why the Correct Answer is Right**
The correct answer is due to the failure of a process called meiosis, which is the type of cell division that occurs in the production of gametes (sperm and eggs). Normally, meiosis involves the reduction of the chromosome number by half, resulting in 23 chromosomes in each gamete. However, in the case of Down syndrome, there is a non-disjunction event during meiosis, where the pair of chromosomes 21 fails to separate properly, resulting in an extra copy of chromosome 21 in the resulting gamete. This can lead to the fertilization of an egg by a sperm with an extra chromosome 21, resulting in a child with Down syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Down syndrome is not the result of a single genetic mutation, but rather the result of a chromosomal abnormality. Additionally, the risk of Down syndrome increases with the age of the mother, with women over the age of 35 having a higher risk of having a child with Down syndrome.

**Correct Answer:** D. Non-disjunction during meiosis.