- If I have a "Am I doing it right" question that pertains to the topic above and is not covered by any of the existing answers, I can't post it as New answer 4: you can also do D, as I am not 100% positive it is a correct solution.
If you just have a conjecture, that this could be another answer, then it's not ready for posting (we want to have quality and facts). You have to prove that your answer works (or need to have anecdotical reference) first.
In case you just want to do a shot in the dark and let someone else (e.g. the OP) prove it, you just take the risk, that your answer will be downvoted (and possibly deleted) for being incorrect.
If you have a very similar question, and don't know the solution D already, you should post a new question, and explain why the solutions you found for the existing question don't fit your needs.
If you already know that a new question would be marked as a duplicate of another one, and you know an alternative answer, add your answer to that original question rather than posting a new question and self answer it.
If you see a question you answered, and it's closed as duplicate, move your answer to the duplicate target. Preferably also delete your answer from the duplicated question if you can (i.e. the OP didn't mark it as accepted).
These are the correct and preferred solutions for the situaitons mentioned above. We want to have as much relevant information in one place as possible.
Note that a question that tends to have many possible solutions as an answer, it should probably be closed as too broad (needs more focus), or for lack of details.