Final answer:
Estonia is the country through which the Narva River flows, draining Lake Peipus and flowing into the Baltic Sea.
Step-by-step explanation:
The country with the Narva river that drains Lake Peipus and flows into the Baltic Sea is Estonia. The Narva River forms a natural border between Estonia and Russia, and after emerging from Lake Peipus, it continues its journey until it reaches the Gulf of Finland, part of the Baltic Sea. The Baltic states, including Estonia, Latvia, and Lithuania, have transitioned from Soviet rule to independence, and have become part of Western alliances such as the North Atlantic Treaty Organization (NATO) and the European Union (EU).