Because eye colours is mainly depend upon a pigment called melanin .
Therefore it is genetically different .

Genetic variation refers to the differences that can be observed in the genetic makeup of individuals within a population. It occurs due to the presence of different alleles (alternative forms of a gene) that can result in variations in traits or characteristics. Among the given options, the example of genetic variation is option 'D' - Two children having different eye colors.

Eye color is a heritable trait that is determined by multiple genes. These genes code for the production of pigments, such as melanin, which contribute to the color of the iris. Different combinations of these genes can result in variations in eye color. For example, one child may inherit a combination of genes that results in brown eyes, while the other child may inherit a combination that leads to blue eyes.

Explanation:
Below is a detailed explanation of why option 'D' is an example of genetic variation:

1. Genetic Basis for Eye Color:
Eye color is determined by the interaction of multiple genes. These genes influence the production, distribution, and amount of pigments in the iris, which gives the eyes their color. The two primary pigments responsible for eye color are eumelanin (brown/black) and pheomelanin (yellow/red).

2. Inheritance of Eye Color:
Eye color inheritance is a complex process involving the combination of genetic variants from both parents. The specific combination of gene variants inherited by an individual determines their eye color. The genes responsible for eye color can have multiple alleles, or alternative forms, which contribute to the genetic variation observed.

3. Different Eye Colors in Siblings:
In the given example, two children have different eye colors. This can occur if they inherit different combinations of alleles for the genes involved in eye color determination. For instance, if both parents carry alleles for both brown and blue eye colors, their children have the potential to inherit different combinations of these alleles, resulting in different eye colors.

4. Significance of Different Eye Colors:
The presence of different eye colors within a population is a clear example of genetic variation. It highlights the diversity that exists in the genetic makeup of individuals, even within a family. This genetic variation is essential for the survival and adaptation of species, as it allows for the potential to adapt to different environments and respond to changing selection pressures.

In conclusion, the example of two children having different eye colors (option 'D') is an example of genetic variation. It demonstrates how different combinations of alleles inherited from parents can result in variations in traits, in this case, eye color. Genetic variation is a fundamental aspect of evolution and ensures the diversity and adaptability of a species.