> "6. Considered as subsets of the real line, this term describes
any
> single point or countable set, any interval that includes its
> endpoints, and any finite union of such sets."
>
> So, the question is indeed flawed, as there do exist sets which
are
> countable but not closed.
It seems as though including the words "single point" is an
assurance that "closed" makes sense as an answer, although I'm not
sure if there are other answers that make sense as well.