The best explanation of the major source of state government power during the period of dual federalism is:
the reserved powers given to states by the Tenth Amendment.
Dual federalism, which prevailed in the United States from approximately the founding of the country until the New Deal era, emphasized a clear distinction between the responsibilities and powers of the federal and state governments. The Tenth Amendment states that powers not delegated to the federal government are reserved to the states or the people, which reinforced state authority during this period.