Final answer:
The 1991 Canada-U.S. Air Quality Agreement is an agreement between Canada and the U.S. that addresses air quality issues by setting a framework for cooperative efforts to reduce air pollution.
Step-by-step explanation:
The 1991 Canada-U.S. Air Quality Agreement is D) an agreement addressing air quality issues between Canada and the United States. It was established to tackle transboundary air pollution problems, which were particularly concerning for both nations because pollution does not recognize international borders. The agreement created a framework for the countries to collaborate on reducing air pollution, setting the stage for further cooperation on environmental issues affecting both nations.