Option D is correct because the defining characteristic of federalism is the division of governmental power between different levels of government. The context clearly states that 'Federalism is a system of government in which the power is divided between a central authority and various constituent units of the country.' It further explains that 'Usually, a federation has two levels of government' and that 'Both these levels of governments enjoy their power independent of the other.'

Option A is incorrect because in a federal system, state governments have powers of their own for which they are not answerable to the central government - the powers are not merely 'given' by the national government. Option B describes the separation of powers among branches of government (legislature, executive, judiciary), which is different from federalism. Option C describes a feature of democracy, not specifically federalism.