There are six key concepts of classical liberal philosophy that are commonly mentioned by the liberal writers, these are; individualism, freedom, natural law, spontaneous order, rule of law and limited state. According to classical liberal writers, social phenomena can only be understood through the study of the individual behavior and this gives the theory its voluntaristic characteristic. The classical liberals emphasize the importance of freedom; protection from intrusion and interference by others. The natural law denotes the customs and practices that serve as the general conditions to be preserved. Another key concept used by liberals is the spontaneous (natural) order which is composed of informal tacit contracts such as customs or conventions. The state is a set of governmental institutions and is a political entity.  According to classical liberals, the sovereign (state) has three duties; (1) protection of its people from invasion of other states, (2) protection of each individual in society from oppression of other member(s) including protection of property and (3) provision of public services and sustainability of the societal institutions important for both individual freedom and peaceful coexistence. The correct answer is D.