Hello! The largest mass extinction in Earth's history occurred about 250 million years ago, right before the dinosaurs appeared. Devonian Period ended about 355 million years ago, which was before the mass extinction occurred. Therefore, C is eliminated. The Cambrian Period came before Devonian Period, ending around 485 million years ago, which also eliminates A. Ordovician Period ended about 445 million years ago, which also before the mass extinction. B is gone, which leaves us to the last available answer choice being D. The Permian Period ended around 250 million years ago, which was the time of the mass extinction. The largest mass extinction even occurred at the end of the Permian period. The answer is D: Permian.