**Core Concept**
Hardy-Weinberg law is a fundamental principle in population genetics that describes the genetic equilibrium in a population. It states that the allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. This concept is crucial in understanding the dynamics of genetic variation and its maintenance in a population.

**Why the Correct Answer is Right**
The Hardy-Weinberg law is related to the concept of genetic equilibrium, which occurs when the forces of mutation, genetic drift, gene flow, and natural selection are balanced. In a population at genetic equilibrium, the frequency of alleles and genotypes remains constant over time. This is because the rate of mutation is equal to the rate of gene flow, and the forces of selection and genetic drift are balanced. The Hardy-Weinberg law is often used to predict the frequency of alleles and genotypes in a population, and it has been widely used in human genetics and evolutionary biology.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Hardy-Weinberg law is not related to the concept of genetic drift, which is a random change in the frequency of a gene or allele in a population over time.
* **Option B:** This option is incorrect because the Hardy-Weinberg law is not related to the concept of gene expression, which is the process by which the information encoded in a gene is converted into a functional product.
* **Option C:** This option is incorrect because the Hardy-Weinberg law is not related to the concept of epigenetics, which is the study of heritable changes in gene function that occur without a change in the underlying DNA sequence.

**Clinical Pearl / High-Yield Fact**
The Hardy-Weinberg law is a fundamental concept in population genetics, and it has been widely used in human genetics and evolutionary biology. It is essential to understand the principles of genetic equilibrium and the factors that influence it in order to interpret genetic data and make predictions about the frequency of alleles and genotypes in a population.

**Correct Answer: D.**