Hint: The evolution seen in the case of Peppered Moths or Biston betularia is the first test of Charles Darwin's natural selection and it caught the attention of many scientists during the time of the industrial revolution.
In this case, the assertion statement is not correct, but the reason statement is correct.
During the Industrial Revolution, a directional color change occurs among the peppered moth or Biston betularia due to air pollution during that time. When the air pollution increased, the frequency of dark-colored moths also increased and a reciprocal decrease of light-colored moths was observed. This is called industrial melanism and is the first recorded and experimented case of Charles Darwin's natural selection in action. After the pollution was decreased, the light-colored moths became predominant again.
Bernard Kettlewell first investigated this phenomenon and found out that a light-colored body was to camouflage in a clean environment like Dorset whereas dark-colored bodies helped in camouflage in a polluted environment like Birmingham. This was a selective survival adaptation due to predators which caught dark moths on clean trees and white moths on darkened trees.
This whole phenomenon was an example of directional natural selection and it helped the moths to save their population by adapting to the environment.
So, the correct answer is option (D) "Assertion is incorrect but the reason is correct".
Note: The investigation and observation done by Kettlewell became the canonical example of Darwinian evolution, however, Theodore David Sargent criticized his experiments due to failure in replicating the experiment which leads to general skepticism. But later on, Michael Majerus's seven-year experiment described Kettlewell's experiment in detail and restored it as the most direct evidence of Darwin's natural selection.