An answer is placed in the comment box on your question and it would make an answer. As long as you quote it is it OK to use it?
See Comments at this question for example.
Good idea to make this a question so I can post my comment there as an answer: yes, this is fine. Actually, in order to avoid arguments over whether it does constitute an answer or not, I'll say this:
The fact that the content of an answer is taken from someone else's comment is not, itself, a problem.
This means that you can post the content of someone else's comment as an answer, as long as the answer is appropriate in other ways. It is polite to offer them a chance to post the answer themselves, sure, but there is no requirement to do so.
If you're going to do this, you do need to follow our guidelines on plagiarism, as well as the Creative Commons license that covers all contributed content on SE.
And of course the answer is subject to all the rules and review procedures any other answer would be subject to.
There is no requirement that the answer be community wiki. (Some people would say this because it's not fair to get reputation for someone else's content, but I say if the original commenter wants the rep, they should post their answer as an answer in the first place.)