The correct answer is - extinction.
When all the members of a species die out, thus disappear from the environment, the species has become extinct.
The reasons for extinction are numerous, but while in the past it has only been because of natural reasons, in recent times it is actually the humans with their activities that contribute to the extinction of the species the most.
There's usually multiple fazes in the decline of a species until it goes extinct, with the last one before extinction being ''endangered'', and from what has been seen so far, once a species reaches that faze, it is almost impossible to recover, but instead it goes extinct.