While all of these choices are actually true, the best choice here is C. Extinction is, in fact, a natural process that has been going on as long as there has been life on the planet; this is important because it allows space and resources to become available as new species arise. Research indicates that in the past extinction has proceeded at a slow pace, as has the development of new species. However, occasionally a planetary-scale disaster such as a large meteorite strike created a phenomenon known as a mass extinction event. Mass extinctions completely destabilised the environment, but were followed by periods of intensive evolution.
The issue at the moment is that the current extinction rate is much higher than would be considered normal. This has led to fears that the extinction rate itself could destabilize the environment. While past history indicates that if this happens, new species should arise, but it will take many thousands of years, and the human race is at risk of being one of the species that becomes extinct in the process.