The concepts of (a) the strong sovereign (the “Leviathan”), (b) sovereign states, and (c) the social contract       were three of Hobbes’s important contributions to realism. As also onderstood from the information given, the correct answer is C.

Information related to the other two options as follows;

Politics as a non-zero-sum game:  The current international system includes aspects of idealism. There are communities and cooperation, often expressed in modern international institutions such as the European Union, the Organization for Security and Cooperation in Europe, or the North     Atlantic Treaty Organization. Idealism bases elements of its theory on politics as a non-zero-sum game; states can benefit by cooperating with other states, and by tolerating difference. Mutual     gains, especially economic and security gains, then become more meaningful and significant than entering conflicts with other states.  

A Self-Help System: There are two basic principles underlying neorealism: first, the international system’s ordering principle is anarchy: there is no such thing as a world power superior to states. This results in an international system that is essentially a self-help system, with autonomous states that are functionally similar actors, each of which must always be prepared to defend or protect itself.