If the most industrialized countries all signed a climate-change treaty with the goal of keeping the sea at its current level, which would be a free rider benefiting from a positive externality of the treaty

The answer is "a small, poor island country in the Pacific".

A positive externality is an advantage that is delighted in by an outsider because of a monetary exchange. Third parties incorporate any individual, association, property proprietor, or asset that is in a roundabout way influenced. A free rider is somebody who needs others to pay for a public good however plans to utilize the good themselves.