The most widely used system of government in the world is this one, in which the national government is supreme, and states and/or provinces exercise only the powers that the national government chooses to give them

A. Unitary System

B. Federal System

C. Socialist System

D. Confederation

a. the unitary system

Explanation:

wikipedia defines the unitary system as:

"A unitary state is a state governed as a single power in which the central government is ultimately supreme. The central government may create administrative divisions. Such units exercise only the powers that the central government chooses to delegate."