The correct answer is option 4, the North Equatorial current. This current is responsible for the increase in temperature in western Europe. The North Equatorial current is a warm ocean current that flows from east to west in the tropical region of the Atlantic Ocean. It brings warm water from the tropical regions towards the western coast of Europe. As the warm water from the North Equatorial current travels towards Europe, it helps to increase the temperature of the surrounding air. This is why western Europe experiences milder winters compared to other regions located at similar latitudes.

Alert - correct answer should be option 2, the Gulf stream. The Gulf stream is the primary current responsible for the increase in temperature in western Europe. It is a strong warm ocean current that flows from the Gulf of Mexico to the North Atlantic Ocean, carrying warm water towards the western coast of Europe. The Gulf stream helps to moderate the climate of western Europe, leading to milder winters and higher temperatures compared to other regions at similar latitudes.