The first known mass extinction is in Paleozoic era. It was caused by oxygen. The oxygen was produced by some types of bacteria performing photosynthesis. This event wiped out most of the bacteria that used anaerobic respiration (did not use oxygen).
Therefore, the option that explains the first known mass extinction is: "The first known mass extinction is in Paleozoic era. It was caused by oxygen. The oxygen was produced by some types of bacteria performing photosynthesis. This event wiped out most of the bacteria that used oxygen."