**Core Concept**
A ring chromosome is a type of abnormal chromosome that results from a deletion of a segment of a chromosome and subsequent fusion of the remaining chromosome ends. This creates a circular structure instead of the typical linear chromosome. Ring chromosomes can be found in various types of cells and are often associated with genetic disorders.

**Why the Correct Answer is Right**
Ring chromosomes are formed through a process called end-to-end fusion, where the broken ends of a chromosome fuse together, resulting in a circular chromosome. This can occur due to errors during meiosis or mitosis. Ring chromosomes can be stable in some cells but may lead to chromosomal instability and genetic abnormalities in others. The presence of a ring chromosome can disrupt gene expression and lead to various clinical manifestations.

**Why Each Wrong Option is Incorrect**
**Option A:** Deletion - A deletion is a type of chromosomal abnormality where a segment of the chromosome is lost, but it does not result in a ring structure.

**Option B:** Inversion - An inversion is a type of chromosomal rearrangement where a segment of the chromosome is reversed end-to-end, but it does not result in a ring structure.

**Option C:** Translocation - A translocation is a type of chromosomal rearrangement where a segment of the chromosome breaks off and attaches to another chromosome, but it does not result in a ring structure.

**Clinical Pearl / High-Yield Fact**
Ring chromosomes can be associated with various genetic disorders, including growth retardation, developmental delay, and increased risk of cancer. A key clinical feature of ring chromosome 20 is a characteristic facial appearance, including a long and narrow face, a pointed chin, and a high-arched palate.

**Correct Answer:** D