Based on the description provided, Nigeria could BEST be categorized as a Mixed economy. This classification is due to the presence of both market-driven elements (such as the ability for individuals to start their own businesses and choose their careers) and government interventions (like tariffs on imported goods and the government's significant role in the oil industry).
For the second question, the statement that accurately reflects the concept of mixed economic systems is:
Mixed economies combine elements of both market and command systems, with government intervention and market forces influencing resource allocation to varying degrees along a continuum.
This statement correctly captures the essence of mixed economies, which lie between pure market and pure command economies, involving a blend of government regulation and market-driven forces.