Your answer is C: A rapid increase in the number of related species
It would be an oncogene, these can be inherited mutations of proto-oncogenes that cause the oncogene.
Jellyfish are forever but fish ain't.
Plus, jellyfish have those tentacle like things attached to thier body and you might confuse em for octopus for that very reason..
but you should really evade confusing em as an octopus cuz octopus might take it as an insult and the next thing you know will be a Kraken taking your house down.. which is pretty tragic :'(