Is this an example of natural selection? <(^_^<)10 points for the best answer(>^_^)>?

Without a clear selective advantage for losing stings, it's hard to say it's due to natural selection. That is, it's not clear why stingless jellyfish would be better off, reproductively, than those with stings. We could speculate, but without data to back it up, that's not really good enough.

Without more evidence, we can't really say whether it's due to natural selection or genetic drift. Drift is a real possibility because there would have been a strong founder effect when the population became isolated. In small populations, neutral mutations can become fixed through random chance. That is, if a "stingless" mutant allele arose or was present in the small founding population, it could have become the only allele in the population just due to dumb luck.

Possibly, but there's not enough information to go on. What benefit to the animal would losing the nematocysts have? Is the possession of the powerful nematocysts genetically determined, and if so, what selective pressure might lead to the loss of these things? Could it be something else - for instance, could the development of nematocysts be dependant upon some factor that is not found in this lake?

The isolation of the lake has allowed the jellyfish to develop on their own and become a unique species. They've lost their stingers through natural selection / evolution because they don't have the need for them anymore.

no