A is the dying out of a large number of species on a global scale

Extinction event or sometimes known as Mass extinction

Explanation:

These events typically are typically widespread and rapid decrease in the biodiversity (which takes into account spread and existence of current species). The worst mass extinction was Permian–Triassic extinction where over 90% of species of organisms vanished.

increased stability in the completion of fluorine's outer shell.

it is the most commonly accepted one