Final answer:
Fascism is the ideology most associated with authoritarian rule, exemplified by the regimes of Mussolini in Italy and Hitler in Nazi Germany, which centralized power in the hands of a single leader or party, suppressed opposition, and promoted aggressive nationalism.
Step-by-step explanation:
The ideology associated with authoritarian rule and the concentration of power in the hands of a single leader or ruling party is Fascism. Fascism characteristically upholds the nation, or the state, above all else, seeking to consolidate power under a single ruler or elite. The ideology promotes a totalitarian way of governance, marked by strong nationalism, suppression of opposition, and often an aggressive foreign policy. Noteworthy examples of fascist regimes include Benito Mussolini's Italy and Adolf Hitler's Nazi Germany, both of which sought to radically transform society to align with their nationalistic and totalitarian ideals.
Additionally, while not all forms of Communism are inherently authoritarian or totalitarian, in practice, several communist states have also concentrated power in this way. One example is the Soviet Union under Joseph Stalin, where the Communist Party controlled nearly all aspects of life and used brutal tactics to maintain its power.