<span>D) Vaccines allow for building immunity without harmful exposure because not all viruses are safe to be exposed to.</span>
The answer is Permian-Triassic extinction.
The Permian-Triassic extinction was 252 million years ago. It is known as The Great Dying because about 95% of all species died out due to environmental changes (volcanic eruptions, dry climate, melting of ice sheets, etc). This is also the only event in which many insect species wiped out.