Final answer:
Estonia is the country that was once part of the former Soviet Union and is now a member of both the EU and NATO.
Step-by-step explanation:
The country that was part of the former Soviet Union and is now an EU and NATO member is C) Estonia. After gaining independence from the Soviet Union in 1991, the Baltic states of Latvia, Estonia, and Lithuania shifted their focus toward Western Europe for trade and development. All three Baltic Republics were welcomed into the North Atlantic Treaty Organization (NATO) and the European Union (EU) in 2004. They have made significant progress in transitioning to market economies with democratic forms of government, despite facing global economic challenges, such as the downturn in 2008.