Question

what justifications are put forward by realism that the states are the prime actors international Relations

Answer 1

JUSTIFICATION THAT PUT FORWARD THE STATE A PRIME ACTORS IN INTERNATIONAL RELATION BY REALISM -

Realism is basically related with International relations and moving around state and state's power. It is also known as political realism. It is contrasted by idealism or liberalism which tend to emphasize corporation.
Realist considered states as a prime actor in International Relation who are considered with their own security, act in pursuit of their own National interest and struggle for power.
The basic assumption of realism is the perpectual existence of conflict among nations (prime actor). One of the guiding principle of realism is permanence of the struggle of power among nations. There are various realists who talk about realism in international relations such as Hobbes, Nicolo Machiavelli, George Kennan, Hans Morganthau and many more. But Hans Morganthau known as prominent realist.
What he said about realism, is -

"International political like all other politics is a struggle of power, whatever the ultimate aims of international relation politics, power is always immediate aim.''

Realism is known as one of the oldest paradigms in international political economics (IPF)
According to realism whealth is substantial factor that determines military capabilities.
States are the central actor of International Relation because there is political Monopoly on action sovereign. As states are the prime actors so greater attention is given to the state not above and beyond that.