What is a central assumption shared by all variants of realism in International Relations?

Correct option is C

The correct answer is c) States prioritize power and survival in an anarchic system.
While Realism has evolved into many different branches (Classical, Structural/Neo-realism, Neoclassical), they all share a common "hard core" of assumptions that define the realist worldview.

Information Booster :

The Three 'S's of Realism
Scholars often summarize the shared assumptions of all realist variants using the "Three 'S's":

Statism: Realists believe that sovereign states are the most important actors in global politics. While other groups exist (like the UN or MNCs), they have no independent power and only act as instruments of state interest.
Survival: The ultimate goal of every state is to survive. Because there is no "world police" to protect them, states must be constantly vigilant. Survival is the precondition for all other goals (like wealth or prestige).
Self-Help: In an anarchic system (a system without a central governing authority), states cannot rely on anyone else for their security. This leads to a "self-help" system where states must build up their own military and economic power to protect themselves.

The Concept of Anarchy
It is important to understand that for realists, "anarchy" does not mean chaos or total disorder. It simply means the absence of a higher authority above the state.
In domestic politics, if someone robs you, you call the police (a higher authority).

In international politics, if one state invades another, there is no "Global 9-1-1" that can guaranteed to save the victim.
This lack of a safety net is what forces states to prioritize power—if you are strong, you are less likely to be attacked.