Who is associated with the political system theory in modern political science?

Correct option is C

The political theorist most prominently associated with the Political System Theory in modern political science is c) David Easton.

Information Booster :
David Easton is widely recognized as the principal figure who developed the concept of the political system and the influential "Eastonian Model" (or Input-Output Model) of political life.

The Political System: Easton defined the political system as that set of interactions by which authoritative allocations of values (policies and decisions) are made for a society.

The Input-Output Model: His model describes the political process as a continuous loop:

Inputs: Demands and Support from the environment.
Conversion Process: The political system (government, legislature, etc.) processes these inputs.
Outputs: Authoritative decisions, laws, and policies.
Feedback: The outputs affect the environment and generate new inputs, making the system dynamic and adaptive.

While the other scholars listed are also giants of modern political science (Robert Dahl with polyarchy, Gabriel Almond with structural functionalism, and Harold Lasswell with the definition of politics as "who gets what, when, how"), David Easton is synonymous with the foundational Political System Theory.