An example of a nation-state is best represented by:

A nation-state is defined as a political entity characterized by a distinct geographical territory and a single, cohesive national identity among its inhabitants. The correct choice illustrates a scenario where the population predominantly shares one national identity, which strengthens social cohesion and political stability. This alignment between a single nation and a defined state is a hallmark of nation-states, as it allows for a sense of unity and belonging among citizens, reinforcing the concept of nationhood within the boundaries of a political state.

In contrast, the other options present scenarios that deviate from the classic definition of a nation-state. A state with multiple national identities could lead to fragmentation and conflict, as different groups may vie for recognition and power. A stateless nation refers to a group of people who share a national identity but do not have their own sovereign state, such as the Kurds or Palestinians, which does not fit the definition of a nation-state. Similarly, a region with diverse ethnic groups typically lacks the homogeneity in national identity that characterizes a solid nation-state, often leading to a multicultural environment rather than a singular national narrative. Therefore, the example that aligns with the concept of a nation-state is indeed where all people share one national identity.