Final answer:
Montesquieu most significantly influenced political thought by advocating the separation of powers in government, a principle that shaped the U.S. Constitution and modern democracy. Thus (option C) is right answer.
Step-by-step explanation:
Montesquieu influenced political thought the most by proposing the separation of powers within the government. This concept was a foundational principle in the drafting of the United States Constitution and has been integral to the development of modern democracies.
His ideas countered the practice of absolute monarchy and advocated for a balanced governmental structure with distinct executive, legislative, and judicial branches to prevent any single entity from gaining too much power. Thus (option C) is right answer.