The statement is - C. Volcanic eruptions have prevented several mass extinctions by cooling Earth's climate.
The volcanoes and their activity have resulted in numerous positives on the planet Earth, being very beneficial for the life as a whole and eventually for the human kind.
The volcanoes though also had some very negative influences, and despite being one of the factors that enabled the life on Earth, they never prevented a mass extinction on it, instead they are responsibly for some of the mass extinctions. The biggest mass extinction dubbed as the ''Great dying'' has been caused by the activity of a super-volcano that existed in what is now Siberia. It managed to wipe out almost all life on the planet. Also, when the asteroid collided with the Earth around 66 million years ago, the volcanoes too contributed largely to the mass extinction that wiped out the dinosaurs. Even in more recent geologic times, the Toba volcano almost wiped the humans from the face of the Earth.