When an animal has a feature that helps them to disguise, in this case specifically making the animal blend into the surroundings, this is called a camouflage. A camouflage is a result of a process by which, under the pressure of natural selection, a feature gets prevalent in a species in order to make this species more fitted to the environment. In this case, the penguins are safer in their environment with this feature. This process is called adaptation, and this is a camouflage adaptation.
Therefore, the correct answer is D. adaptation.