Answer:
a. directional selection
Step-by-step explanation:
Directional selection is a type of natural selection wherein one extreme trait is favored and the individuals with that extreme trait are at survival and reproductive advantage.
In the given example, dark-colored rats are favored by natural selection over the light-colored rats. This occurs because the rats with dark skin color have better survival rats in the prevailing surrounding. Since one extreme trait (dark skin color) is favored over the other (light skin color), the rat population is undergoing directional selection.