What is defined as a legal entity with a government managing a population in a specific territory?

The correct answer is "state" because it refers to a legal entity that possesses a defined territory, a permanent population, a government, and the capability to enter into relations with other states. The term "state" in international relations is specifically used to describe a political organization that holds sovereignty over its territory and the people within it.

While the term "nation-state" closely relates to this definition, it specifically refers to a state that is associated with a specific nation or cultural group, emphasizing the concept of national identity. "Ethnic group" denotes a community of people who share common cultural ties, such as language, culture, or ancestry, but does not imply governance over a territory. A "confederation" describes a union of independent states that delegate certain powers to a central authority while retaining their sovereignty; it does not refer to a singular legal entity with full governance over a defined area. Thus, "state" is the most accurate term for a legal entity under international law with governing authority over its territory and population.