The character Mowgli was nurtured, just not by humans. Therefore, it is not answer C. Also, he was not "rejected by society" or an "outcast" so it is not A or B. The answer is D. Mowgli is a man who has grown up according to the rules and customs of the jungle, as it says, BUT even though he is a man, his ideas of etiquette and communication are those of animals, rather than people. Because of this, Mowgli does not understand "the law of men" which refers to how humans behave, which causes him to not be able to "fit in" with society. It's basically a cultural barrier. The whole theme of the story is that he is still human, even if his way of life is different. Sometimes, certain cultures look down upon others because they have different customs, which is rude and ignorant. This story has a theme of recognizing people as humans in spite of cultural differences.