The correct option is D nationalism
Nationalism: It is a belief system which instills a sense of common identity among the members of a nation. National flag, national symbol, national anthem, etc. play an important role in developing and strengthening the idea of nationalism. The term Nationalism refers to develop sentiments or views towards one’s own country. It is the knowledge regarding the social, political and economic aspect of the society or of the nation to promote its interests or beliefs. Hence, Option A is correct. The rest of the ideologies are not central to the formation of a nation-state, hence, incorrect.