**Core Concept**
Down's syndrome is a genetic disorder caused by an extra copy of **chromosome 21**. This condition is characterized by distinct physical features and intellectual disabilities. The underlying genetic principle involves **trisomy 21**, where an individual has three copies of chromosome 21 instead of the usual two.

**Why the Correct Answer is Right**
Since the correct answer options are not provided, I will explain the general concept. Down's syndrome is typically associated with **trisomy 21**, which occurs when there is an extra copy of chromosome 21. This extra genetic material alters the course of development, leading to the characteristic features of Down's syndrome. The **meiotic nondisjunction** of chromosome 21 during gametogenesis is a common cause of this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's challenging to provide a detailed explanation. However, any option that does not involve **trisomy 21** or a similar genetic mechanism would be incorrect.
**Option B:** Similarly, without the specific option, it's difficult to provide a detailed explanation. Any option that does not accurately describe the genetic cause of Down's syndrome would be incorrect.
**Option C:** This option would be incorrect if it does not involve a genetic mechanism or **chromosomal abnormality**.
**Option D:** This option would be incorrect if it does not accurately describe the genetic cause of Down's syndrome.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Down's syndrome is associated with an increased risk of **congenital heart defects** and **intellectual disability**. Early diagnosis and intervention can significantly improve the quality of life for individuals with Down's syndrome.

**Correct Answer:** Correct Answer: D. A chromosomal disorder.