Of the 5 major mass extinction events, which was most severe?

Permian-triassic Extinction: 250 million years ago                                           The largest mass extinction event in Earth's history affected a range of species, including many vertebrates.

Explanation:

hese five mass extinctions include the Ordovician Mass Extinction, Devonian Mass Extinction, Permian Mass Extinction, Triassic-Jurassic Mass Extinction, and Cretaceous-Tertiary (or the K-T) Mass Extinction.

The largest extinction setback was the Permian-Triassic extinction, also called the “Great Dying,” some 252 million years ago. Up to 96% of all marine species and 70% of terrestrial vertebrate species went extinct.

if you listed your answers right then i believe your answer is b.