**Core Concept**
Polymorphism in genetics refers to the occurrence of two or more clearly different forms or types of a gene in a population. This phenomenon arises due to variations in DNA sequences, which can lead to differences in protein structure and function.

**Why the Correct Answer is Right**
Polymorphism plays a crucial role in understanding genetic diversity and disease susceptibility. The most common type of polymorphism is single nucleotide polymorphism (SNP), where a single nucleotide is changed. This can affect gene expression, protein function, and ultimately, disease risk. For example, the APOE gene polymorphism is associated with an increased risk of Alzheimer's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of polymorphism being referred to.
**Option B:** This option is incorrect because it is too broad and does not provide any specific information about the concept of polymorphism.
**Option C:** This option is incorrect because it is a definition of heterozygosity, not polymorphism.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that polymorphism can affect gene expression and disease susceptibility, highlighting the importance of genetic diversity in understanding human disease.

**Correct Answer:** D.