Final answer:
A nation-state, best described by Option A, is an independent group of people living in a defined territory with an organized government, sharing a common culture and identity.
Step-by-step explanation:
A nation-state is a concept that combines elements of a nation (a group of people with a common identity based on culture, language, and history) with those of a state (a political entity with a defined territory and an organized government). To answer the student's question directly, Option A, which states that a nation-state is 'An independent group of people living in a defined territory with an organized government' best describes a nation-state. Nation-states typically have a centralized government that exercises authority over the territory, creates laws, and maintains a monopoly on the legitimate use of force. Prominent examples of nation-states include countries like Japan, where the majority of the population shares a common heritage and the government actively preserves the national cultural identity.