The correct answer is - A. the cyclical warming of the Pacific Ocean near the equator.
El Nino is a natural phenomena that still can not be explained by the scientists, why is it appearing, why on that particular place... It is an event that takes place in the Pacific Ocean, near the Equator, when the water of the Pacific Ocean in its eastern part starts to warm up, and the result of that are also warm air masses. As the water and air warm on that side of the Pacific, on the other one, it is the total opposite, the water and air masses are cooling down.
Even though El Nino is only appearing in the Pacific, it has a global effect on the climate in the years when it is active, and very often prolonged dry summers or winters, as well as heavy rainfalls and storms, depending on the part of the world, are associated with it.