When found an external page that explains well and specifically the point of a question in stackexchange, which of the following options is preferable and why?
1) Making a comment to the question with a link to the external page with the answer.
2) Making an answer to the question with the contents of the external page (and of course, a reference to the source).
3) Close the question as it is a signal that there wasn't enough previous research by the original poster, as pointed out by user3856370.
I think that replicating the contents of the external site in stackexchange only creates redundancy in the internet (assuming you don't need to add any clarification or comment of your own). However, there might be good reasons to post it in stackexchange anyway.