Final answer:
Estonia is the country with its capital city and major port in Tallinn, which is a significant economic center on the Baltic Sea.
Step-by-step explanation:
The country that has its major port and capital city in Tallinn is Estonia. Estonia is one of the Baltic Republics, along with Latvia and Lithuania, which gained independence from the Soviet Union in 1991. After independence, Estonia, like its Baltic neighbors, transitioned to a market economy with a democratic government and focused on integrating with Western Europe. The city of Tallinn not only serves as the capital of Estonia but is also an important economic hub due to its strategic location on the Baltic Sea.