(note: I'm just here as a regular user; please ignore the ♦ - I'm trying very hard not to act in a moderation capacity here; my answer here is not binding, etc)
Copying something from elsewhere (esp. passing it off as your own) is plagiarism; that is pretty much the definition. In this case, the question is such a 1-liner that it could quite conceivably be 2 people genuinely asking the same question - there are, after all, only so many ways of asking the same question. Personally, I think this also demonstrates that perhaps islam.SE should encourage a bit more "meat" to questions, rather than just a 1-liner. What have they considered? What have they looked at? What views have they considered? What conclusions have they drawn already? etc. If a question fits on one line, it probably needs more work!
When it comes to something larger (for example, the answer that was copied verbatim from another site), it isn't enough just to indicate where it came from. That can lead to all sorts of problems with copyright etc. A better approach is usually to actually write your own answer, perhaps referencing external sources (maybe with a minimal citation from the external site). That makes it clear that the answer is your own, and didn't just come from Google.
If the person answering has to add sources, why doesn't the questionnaire?
Well, copying a question from another site also probably isn't a good thing. In this case, the question is so minimal that it is hard to say definitively whether it was copied, versus two people independently having the same question.