Final answer:
The concept of a nation-state is most important for border security as it directly deals with a sovereign entity's control over its territorial borders.
Step-by-step explanation:
In the context of the issues listed, the concept of a nation-state would be most important for border security. A nation-state is a sovereign entity with defined territorial borders, which exercises control over its land and people. Border security directly pertains to the nation-state's ability to regulate who and what crosses its borders. Issues such as air pollution, job outsourcing, and the spread of epidemic diseases are also significant, but they do not primarily revolve around the concept of a nation-state as border security does. Air pollution and epidemic diseases often require international cooperation as they can cross borders, whereas job outsourcing is more related to global economics than the notion of a nation-state itself.