Hello Majid, The first example would be the most common and natural way of speaking: "Who wrote this article?" For example, you just read a very interesting article in a newspaper or a magazine and you are interested in the identity of the author, and so you would ask: "Who wrote this article? I would like to read (or see) more of his/her work." The second example would be used much less common and could be used to express emphasis. For example, say there is an article or a book or manuscript with an unknown author, or the writing is from a time long past in history and therefore there are several theories as to who is the true author. After presenting evidence of different possible authors, you might conclude by saying "WHO then, did write this article?" You are adding the word "did" to make a greater emphasis, to express that we do not really know the answer. This is especially true concerning manuscripts from long ago in history. I hope that this helps, if not, please ask more.

Who wrote this article is correct. it's a grammatical exception, of some sort. usually, it is "who + did + verb + object". But here it is simply "who wrote this article?"