Final answer:
The concept of a nation-state is most important in the context of border security, as it concerns a country's ability to maintain its sovereignty and control over its territorial boundaries.
Step-by-step explanation:
The concept of a nation-state would be most important in issues related to border security. A nation-state is a territory where the cultural boundaries match up with the political boundaries; in other words, a region that is both a physical space and a locus for a unified cultural identity. It is an area regarded as a sovereign entity, which controls its affairs without external interference, and border security is a critical aspect of maintaining this sovereignty. Border security involves measures taken by a country to regulate and monitor its borders, ensuring the security and integrity of its territory, which is essential for the survival of a nation-state. Therefore, among the options given (air pollution, border security, job outsourcing, and spread of epidemic diseases), border security is the issue where the concept of the nation-state is most significant.