Final answer:
The most distinctive principle of U.S. democracy is federalism, which distributes power between the national and state governments, making it unique from other forms of governance.
Step-by-step explanation:
The principle most distinctive of the structure and character of U.S. democracy as opposed to other forms of government is federalism (D). Federalism in the United States is the apportioning of power between two levels of government: national and subnational (states). This system allows for a balance and distribution of power that is unique compared to other forms of government like unitary or confederal systems, where power is either centralized or dispersed among member states without a strong central government. Moreover, none of the other options - separation of powers (A), universal suffrage (B), and rule of law (C) - are as uniquely American since they are features found in many other democracies as well.
Federalism, as seen in the U.S., is a hallmark of American democracy because it creates a dual system of government where sovereignty is constitutionally divided between a central governing authority and constituent political units. This allows states to have their own laws, policies, and systems of government while still being united under a federal system that governs the whole nation. Other important characteristics such as separation of powers and the rule of law support the operation and stability of democracy, while universal suffrage ensures the democracy remains representative of its citizens.