The correct answer is - a. volcanic eruptions.
The volcanic eruptions had been one of the crucial factors throughout the Earth's history in shaping its climate. The bigger volcanoes, or the massive super-volcanoes, have the ability to release enormous amounts of Greenhouse gasses into the atmosphere, as well as releasing huge quantities of ashes into the atmosphere.
The Greenhouse gasses from the volcanoes have the ability to warm up the temperature on a global scale. On the other side, the ashes have the counter effect, as they can make a layer in the atmosphere around all of the globe, which will block the Sun, and result in sudden cooling off on a global scale.
