Rank the following from most to least devastating: (a) Ordovician, (b) Permian, (c) Cretaceous, (d) Pleistocene.

From the most to least devastating:

1. Permian

2. Ordovician

3. Cretaceous

4. Pleistocene

Explanation:

Permian, the world had at the time two continents and a global ocean. 96% of marine species and 70% of terrestrial species ceased to exist, that's what make it at the end the largest mass extinction in Earth.

Ordovician started with a considerable extinction that severely lower the number of trilobite species, and end up with the extermination of 60% or marine species.

The Cretaceous period ended with a large mass extinction in which many species in addition to large marine reptiles were wiped out.

The end of the Pleistocene period coincides with the end of the recent glacial period and has at least about 70% of the fauna still living today.