The correct answer is option 3, the Pacific Ocean. El Nino is a weather phenomenon that occurs in the tropical Pacific Ocean, specifically in the region near the equator. During El Nino, the surface water temperature of the Pacific Ocean becomes unusually warm, causing changes in weather patterns worldwide. This warming of the ocean affects the atmospheric conditions and can lead to droughts, floods, and other extreme weather events in various parts of the world. It is important to note that El Nino does not occur in the Atlantic Ocean, Indian Ocean, or Mediterranean Sea. Therefore, options 1, 2, and 4 are incorrect.