Answer: The correct answer is- d. competition.
Competition can be described as a biological interaction between individuals belonging to same ( intraspecific) or two or more different species ( interspecific) for territory, food, and other resources.
Dogs bark at unknown individuals, when they approach their house. By this behavior, they exhibit defense for their territory. This behavior is exhibited because dog is fearful and perceives strangers as a threat to their territory.
Thus, option d) is the right answer.