The correct answer is option 2, which states that the Baltic Sea ports remain open for trade during winter because of the North Atlantic Drift, a warm ocean current that flows in the region. The North Atlantic Drift is a powerful current that brings warm water from the Gulf Stream into the North Atlantic Ocean, which helps to moderate the climate in the northern parts of Europe, including the Baltic Sea region. This warm water helps to prevent the ports from freezing over completely during the winter months, allowing trade to continue. This is in contrast to other regions at similar latitudes that are not influenced by warm ocean currents, where ports may freeze over and trade is severely impacted.