**Core Concept**
The Hardy-Weinberg principle is a fundamental concept in population genetics that describes the expected frequencies of alleles in a population over time. It assumes a large, randomly mating population with no mutation, genetic drift, or selection pressure.

**Why the Correct Answer is Right**
The Hardy-Weinberg principle states that the allele frequencies in a population will remain constant from generation to generation, provided that certain conditions are met. These conditions include a large population size, random mating, no mutation, no genetic drift, and no natural selection. The principle is expressed mathematically as p^2 + 2pq + q^2 = 1, where p is the frequency of one allele and q is the frequency of the other allele. This equation shows that the frequencies of the two alleles will remain constant over time.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the Hardy-Weinberg principle, which deals with allele frequencies in populations, not the structure of DNA.

**Option B:** This option is not accurate, as the Hardy-Weinberg principle specifically addresses the frequencies of alleles, not the frequencies of genotypes.

**Option C:** This option is not correct, as the Hardy-Weinberg principle is a descriptive concept that explains the expected frequencies of alleles, not a predictive concept that forecasts the outcomes of genetic changes.

**Clinical Pearl / High-Yield Fact**
The Hardy-Weinberg principle is a fundamental concept in population genetics that helps us understand the expected frequencies of alleles in a population. It is essential to remember that the principle assumes a large, randomly mating population with no mutation, genetic drift, or selection pressure.

**Correct Answer: D. Hardy-Weinberg principle.**