This question already has an answer here:
Some questions are being answered quickly in on of the comments; in some cases, the OP even "accepts" (in words, in another comment) the comment-answer.
Here's an example.
The problem is that such questions are stuck forever in the "Unanswered" section, as there's really no need to add a formal answer, and the OP is probably never going to look for new answers, and it is already informative as is for google-comers.
In any meaningful sense - this question is answered.
What can we do with such questions?
I've seen here and there ad-hoc requests such as "please phrase this as an answer so I could accept it"; but that's really specific (and quite annoying). Should we allow OPs to "accept" a comment as an answer (I know this was discussed here, but that's a very old discussion), or should we allow moderators to flag old questions as answered if it's clear that an answer exists and the OP is ok with it? (this is similar to that offer, but is still different)
I know there have been many discussions touching the subject from various aspects, but this is a very specific offer: let moderators flag a question as answered when it has no answers, it has an "answer" in the comments, and it is clear the OP is "accepting" that answer. I have not seen that specific offer here.