All of the options you listed contributed to Russia's struggles in the early 1900s, but the most significant factors related to why Russia lost power and faced upheaval can be summarized as follows:
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Unrest among the poor: There was significant social and economic unrest among the working class and peasantry due to poor living conditions, lack of political rights, and exploitation. This unrest fueled revolutionary movements and weakened the authority of the tsar.
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Russia had a weak king: Tsar Nicholas II is often considered ineffective and out of touch with the needs of the Russian people. His inability to address the issues of his time, resistance to reform, and reliance on autocratic rule undermined his legitimacy and authority.
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Russia failed to westernize: Unlike other nations that adapted and modernized their societies economically and politically, Russia lagged in industrialization and failed to embrace reforms that could have strengthened its government and military.
Although the size of Russia's army could be a double-edged sword, it was not primarily the reason for losing power; rather, it was the combination of social unrest, ineffective leadership, and failure to modernize that led to the decline of Russian power in the early 20th century, culminating in revolutions.
In summary, "Unrest among the poor" and "Russia had a weak king" are likely the most pivotal reasons in this context, with "Russia failed to westernize" also being very significant.