The correct answer is - Gulf Stream.
The Gulf Stream is an ocean current in the Atlantic Ocean. This current is warm, thus it brings in warm and moist air masses, which results in much warmer and stable climate throughout the year on the places that it influences.
The Gulf Stream starts in the southwestern parts of the North Atlantic, and in continues in northeastern direction towards Europe. Halfway to Europe, the Gulf Stream changes in the North Atlantic Drift, and further on the North Atlantic Drift changes into the Norwegian Current, so all extensions from one another.
They manage to make the western coastline of Europe much warmer, to not have any extremes in the temperatures, and also to have a lot of precipitation throughout the year.