I'm surprised to not see another post for this. Sometimes a user will post one question and never respond to anything. An answer is posted that appears to be correct. A year goes by. The post is still "unanswered". Should we flag them with a note to "mark answered"? Or...?
No; don't do that. Nobody except the OP can accept an answer by proxy. Not even ♦ moderators. All you will do is create noise for the moderators to ignore.
If the answer is good, upvote the answer. That will make it clear that it has a sensible answer.