Answer:
externalities that cross national borders and that a single nation acting alone cannot resolve
Step-by-step explanation:
International Externalities is a term that describes the externalities that goes beyond the border of a particular country or nation, therefore to solve such externalities, it will involve more than one nation or country to resolve it.
For example, the is issue of Global warming: a rise in average temperature of Earth's climate system, and Biodiversity are prime example of international externalities.