I'm not suggesting there are any currently on the site (there may be, may be not), but it occurs to me that there's the distinct possibility.
On StackOverflow, as example, an unanswerable question would have to be a question where the posting user essentially lied about the actual results. On StackOverflow, your code isn't producing the correct results, so the answer is whatever change needs to be made to your code to produce the correct results. You can't possibly post an on-topic question that is unanswerable.
The story is different here though.
Here, a requirement for being on-topic is that the code actually work. And the question you ask is "Can my code be better?"
Is "No, I don't see any way to improve this code." an acceptable on-topic answer? I don't think that's very helpful to the site or to the person posting the question--particularly because generally speaking, anyone using this website is likely always writing the best code they can... but similarly, anyone using this site is unlikely to think they know all of the best ways of doing something.
Is it helpful to post an answer along the lines of:
"I think this code is good and I wouldn't make any changes to here. Here are the things I think you did particularly well and why I think these are the best written parts of the code"?
This answer says almost the same thing as "No, I don't see a way of improving the code," but at least affirms that some particularly parts of it were certainly good in the eyes of one answerer.