There are a huge amount of betta species
out there, but maybe they meant tail types?
Also some species of betta do come from small swampy puddles/pools, but that doesn't mean that the water itself is filthy. I think sometimes people get confused about the word 'stagnant' which actually means slow-moving, not dirty. There are probably a million times more microbes and other bacteria in natural bodies of water than there are in our aquariums.
Sounds like a pretty poor quality quiz if you ask me. So I wouldn't get too hot and bothered about it.