Final answer:
A common trade network to share and trade resources
Step-by-step explanation:
The most useful connection for the countries of Scandinavia, Estonia, Latvia, Lithuania, and Poland along the Baltic Sea would be a common trade network to share and trade resources.
This would allow these countries to benefit from each other's resources, promote economic growth, and increase interdependence in the region.
By establishing a common trade network, these countries can foster cooperation, improve their economies, and strengthen their overall regional development.