**Core Concept**
Down's syndrome is a genetic disorder caused by an extra copy of chromosome 21 (trisomy 21). This extra genetic material leads to a variety of physical and developmental anomalies. The condition is often associated with intellectual disability and increased risk of certain medical conditions.

**Why the Correct Answer is Right**
The correct answer is not a mutation in Down's syndrome because the disorder is primarily caused by a chromosomal abnormality, specifically trisomy 21. This means that instead of the usual two copies of chromosome 21, individuals with Down's syndrome have three copies. This extra genetic material disrupts the balance of genes and leads to the characteristic features of the condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while genetic mutations can contribute to the development of Down's syndrome, the primary cause is a chromosomal abnormality, not a mutation.

**Option B:** This option is incorrect because a deletion of chromosome 21 would result in a condition known as monosomy 21, which is not associated with Down's syndrome.

**Option C:** This option is incorrect because a translocation of chromosome 21 to another chromosome is a type of chromosomal abnormality that can contribute to the development of Down's syndrome, but it is not a mutation.

**Option D:** This option is incorrect because a duplication of chromosome 21 would result in an extra copy of the genetic material, which is consistent with the cause of Down's syndrome.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Down's syndrome is not caused by a single genetic mutation, but rather by a chromosomal abnormality. This distinction is crucial for accurate diagnosis and management of the condition.

**Correct Answer:** B.