A government by the nobles

Answer: Option D

One word substitute is Aristocracy.

Democracy: a system of government by the whole population or all the eligible members of a state, typically through elected representatives.
Bureaucracy: a system of government in which most of the important decisions are taken by state officials rather than by elected representatives.
Autocracy: a system of government by one person with absolute power.
Aristocracy: a form of government in which power is held by the nobility.