The correct answer is - The calming down of trade winds in the Pacific, which leads to the buildup of warm water.
El Nino is a natural phenomenon that appears in the southwestern part of the Pacific, and there's still not a conclusive theory about what is the reason that it appears, on every four years usually.
The beginning of El Nino is marked with the calming down of the trade winds in this part of the Pacific Ocean. As the trade winds calm down, the water starts to warm up, while in the same time, the water is cooling down in the opposite part of the Pacific.
El Nino manages to affect the climate all over the globe, but the biggest influence of it is felt in the western part of South America, where there's either long droughts with scorching temperatures, or massive rainfalls that result in landslides and floods.