No answer has been posted to that question.
All that you've received is a comment, and comments are very different from answers. They cannot be accepted, they do not affect the reputation of the contributor, and they are frequently cleaned up by moderators.
If, as in your case, your question has been "answered" by a comment, you have a couple of choices:
You can reply to the user who posted the comment (using the @ reply syntax), inform them that their solution has solved your problem, and ask them to "promote" their comment to an actual answer. Once they've done that, you can accept their answer just as described in the Help Center and the FAQ.
If you have some additional information or code that you can add to the comment, or if you've asked the user who posted the comment to post an answer but they haven't done so within a day or two, you can post an answer yourself. Answering your own question is totally encouraged; it shows that you're an active participant and able to solve problems yourself. There are some restrictions on new users accepting their own answer to avoid poor sportsmanship, but after 8 hours, you can acccept it just like any other. And you should do so! After all, it is the answer and it best solved your problem.
If you do not think the answer to your question would ever be useful to anyone else in the future of the Internet, you can simply delete it. Questions can be deleted by the original poster until they have received at least one upvoted answer. But you should use this power very sparingly: our goal is to build up a comprehensive database of high-quality answers to a diverse array of programming questions. Deleting questions with a correct answer is counter-productive to that goal. Only delete questions where the answer is something like "oops, you made a typo".
A feature to "accept" a comment as if it were an answer has been proposed numerous times, but extensive discussion has concluded that it is not a good idea. It would be encouraging the wrong thing. We want people to post answers, not comments.