So my question is:
can I copy paste that answer?
This is an answer to your question from my perspective, not wearing my moderator's hat.
That particular answer was written by someone else and is their intellectual property, which they've explicitly licensed to Yahoo (via the appropriate Terms of Service) but not necessarily elsewhere. For you to take those exact words and display them elsewhere as if you had written them yourself wouldn't be honest.
I think that you, Ramanujan, are perfectly capable of distilling the differences between the existing answer on the question you link and the answer you have found elsewhere, and the world would be a better place for your efforts.
If you think there is something ineffable about the answer you've found that can't be reproduced by your efforts, a link in a comment would sit better with me than a big block quotation in an answer.
I don't think that any of your options here are wrong in a way that would cause trouble, but copy-pasting content from elsewhere is my least favorite among them.
Here's a checklist to run through when considering this:
this smthg abt zero vector and its uses...
Frankly, I see no reason whatsoever to do what you're doing, but yes, legally and by the rules of Stack Exchange, you can do it (again, if the content isn't copyrighted).
Is it recommended? No. Is it allowable? Yes.
According to a non-mod answer on the mother meta site, it is very much frowned on to copy a Yahoo Answer without attributing it, but it seems perfectly okay from a legal standpoint.
I recommend going through that post and any related posts to be sure, but so long as you cite your answer, you should be fine for the most part.