Classical Realism and Neo-realism as a whole body of Realist paradigm share some common assumptions that are key in understanding security in international politics. These common assumptions are: 1) International system is anarchic; 2) Power is the defining notion of state behaviors in the anarchic international environment; 3) States are the most important units/actors in international politics; and 4) States are unitary and rational actors. Choice A "International corporations are the most important units/actors in international politics." is not a common assumption of Classical Realism and Neo-realism. So the correct answer is A.