Answer:
The best example of a unitary system among the options provided is D) Sparta.
A unitary system of government is characterized by a central authority that holds significant power and makes decisions for the entire country or state. In a unitary system, local or regional governments have limited powers and derive their authority from the central government.
Sparta in ancient Greece is often cited as an example of a unitary system. In Sparta, power was concentrated in the hands of a small group of elite citizens, and the central government controlled all aspects of society, including military, political, and social affairs. Local governments within Sparta had limited autonomy and were subject to the authority of the central government.